Cayden is very good at this. 

Astarion plays with his hair. If he can't make Cayden enjoy it directly, he can at least make it clear he's enjoying himself and hope Cayden likes that. 

Cayden appears to be entirely oblivious to all the drama happening in Astarion's head, but very interested in the head of his dick. 

Astarion wouldn't dare to disagree with his dick being fascinating.

It's very fascinating! Cayden is fascinated by it. 

Cayden might not notice hair petting, but surely he can't miss cut off breathy moans?

He will do more of whatever leads to moans!

Lucky for him,  Astarion is easily pleased when he's trying to make things worthwhile.

The only person Astarion is sabotaging here is himself!

Cayden can't explain this because his mouth is full and also he's not telepathic.

Cayden is really incredible at sucking dick.

He honestly is, and Astarion is not putting it on all that much. 

"There-- like that--"