I'd be more INFURIATED at how incompetent you are at hurting people, except for how EVERYBODY IN THIS COUNTRY EXCEPT ME is terrible at it. Nonetheless, shut up and listen. You're not going to understand, because NOBODY ever understands except ME, but listen anyways.
Torture is not a magic wand that makes people better at things when you apply it. It works great as a magic wand for producing terror and regret, but for that to IMPROVE PERFORMANCE there has to be some kind of AQUEDUCT going from the FEAR LAKE to the RESULTS FARM.
If somebody knows HOW to achieve a result and is just feeling reluctant about the effort or inconvenience involved, telling them they'll get hurt unless they achieve the result works GREAT. If somebody DOESN'T KNOW HOW to do something, you can't TORTURE THEM INTO KNOWING.
I can't take a sixth-circle wizard and tell them that if they don't close the Worldwound in a year, I'll make an artistic production of their suffering such that when they get to Hell their owner will feel impressed and competitive about it, and have them actually close the Worldwound. I tried that just to see if that might work, if maybe all the wizards were just being lazy and unmotivated about actually getting the Worldwound fixed, but no, they actually don't know how to do it, and don't know how to figure out how to do it, no matter how desperate they are.
People need to know, see, feel, that doing one thing leads to more pain, and doing something else leads to less pain. They need to know how to achieve their goal of hurting less.
You can, by means of pain selectively applied, cause your boy to keep trying to write, to keep his hand moving on the paper. If he doesn't know how to write then you can only solve this by teaching him how to write.
Now, often people do know how to do something, and don't want to, and whine that they don't know how. But you know when that whining is, maybe, a bit more believable? When it comes from an eight-year-old boy, especially one whom you've already hurt and that hasn't fixed the problem yet.
It's worth hurting an eight-year-old boy some, maybe, to see if that fixes things, to incentivize them not to try whining as their first strategy. But eight-year-olds aren't paladins, Slappy. It's not that hard to break their resistance. If they're still not doing it after you hurt them a few times, it's because they don't know how, not because they're testing your resolve.
Let me tell you one of the secrets of Hell, Slappy. If you go to Hell carrying high enough rank in Asmodeus's tyranny that the lesser devils there will answer your questions, and ask them, "Why did you hurt that petitioner?", they give reasons like, "Because I thought it would be funny", "So that in the future they will fear more to waste precious ink and be more careful in their copying", "So that this section of the Count's palace shall have an artistic ornament befitting His place in Hell".
If you then go back to Cheliax and ask mortals why they hurt people, they say, "Because he fucked up on his job", "Because she scorned me", "Because they did not show me the respect due my rank."
See the difference, Slappy?
Of course you don't.
I'll spell it out for you: If you ask mortals why they hurt people, they talk about things that ALREADY HAPPENED, while if you ask devils why they hurt people, they talk about things THEY WANTED TO MAKE HAPPEN IN THE FUTURE. Which makes sense, see, because you CAN'T CHANGE THE PAST, so if you were trying to use torture to actually DO ANYTHING PURPOSEFUL it would be something having to do with the FUTURE.
It was at this point, Slappy, that I realized that the reason why mortals can never do anything right with torture is that they ARE NOT TRYING TO DO THINGS except by COINCIDENCE. They're acting out their own anger, frustration, unhappiness, by hurting people. That does, in the end, support Asmodeus's tyranny in the mortal world; for subordinates often do know methods to avoid angering their superiors, and often, one of those methods is to do well at their jobs. But it's not the way of Hell.
What are you doing, Slappy? You're getting angry at your boy's lack of performance and you're hurting him and you feel like that ought to make him better and he doesn't get better so you feel frustrated and you act that out by hurting him more. Do you know what that makes you? It doesn't make you good at Asmodeanism, it makes you BAD AT TORTURE.
You're slapping him on the hand he uses to write. Don't fucking tell me it's because you want to teach him to overcome adversity. When somebody is STRUGGLING TO DO SOMETHING AT ALL and get to ANY rewards at the end of the problem that will teach their brain there's an ALTERNATIVE AT ALL that leads to LESS PAIN, is not when you add on ADDITIONAL ADVERSITY to teach your EIGHT-YEAR-OLD BOY how to be TOUGH. Teach him that by adding adversity to something he can ALREADY DO and punishing him for failure at THAT.
You fucking obviously didn't carefully plan out a torture scheme that would involve slapping his hand, so you could simultaneously teach him how to write at all and how to overcome adversity in the form of hand pain. You acted out your frustration in the first way that came to mind, by slapping his hand, and when your husband pointed out the possible issue you made up the adversity thing AFTERWARDS. That doesn't make you good at Asmodeanism, it makes you BAD AT TORTURE.
But you're not going to understand any of that. So if I had to leave you with a simple rule, it would be this: Only try hurting your boy for long enough that it would break his resolve if he did know how to do the work and was just trying to resist you about that, and if that doesn't work, STOP and go TEACH HIM INSTEAD.
When he finally shows he can solve the problem, do NOT tell him immediately after that now he needs to go do all his work or get hurt, because then you're torturing him into NOT LEARNING THINGS. Give him plainer food until he learns, restore his usual rations after he learns successfully. As soon as he LEARNS TO DO THE WORK BETTER he should notice his life becoming IMMEDIATELY LESS UNPLEASANT. Do not make this connection complicated. Your little boy is even stupider than you are.
And don't tell me you're hurting your boy because it's fun, either. Your boy is somebody you're currently treating as a valuable investment, and you shouldn't throw away that investment because you're flailing to find an excuse for why you're hurting him. But if you want to deliberately stop with that policy and treat him as a fun torture doll instead, then sure, by all means, have fun! Just don't act surprised when he breaks and stops being useful.
PS: I have no idea what your boy means by 'letters moving around'. But what people say when they're in pain often means SOMETHING, especially after you've hurt them enough that they could not reasonably be trying to deliberately annoy you. So possibly consider paying attention to that part too, and asking questions, instead of getting FRUSTRATED and applying pain in response to EVENTS THAT ALREADY HAPPENED IN THE PAST. It doesn't make any sense to you, yes, but 8-year-olds do not always express themselves perfectly the first time. LISTEN TO PEOPLE WHILE YOU TORTURE THEM.