If we all attacked him at the same time, would we win?

Yes, easily in fact, if the dragon just stood there.
More likely he'll realise that, fly out of range, and then burn your weakest kin to death first, one by one, at his leisure.

Even if you found a way to kill him during that, it would be a pyrrhic victory. Your best alternative would be to flee, which might save some of you individually but would guarantee the destruction of your tribe and the deaths of almost all your children. 

If you carefully planned an ambush, choosing buff spells in advance, you could win more cheaply, probably.

And no doubt everyone else will realise that too.
He knows what is required of him.

"We will submit to him, then."
Bluff +2

"The Chief says that we will submit to you."
Truth.

1d20 + 2 = 13
1d20 + 10 = 21.

He's planning to butcher you in your sleep.

Figures.

"Please instruct the rest not to get involved, then."

He knows full well what is about to happen.
Hadregesh's service does not require that he deceive himself at all about the ways of the world, as so many other gods do.

He'll just nod.

He approaches nonchalantly.
Bluff +10

Sense Motive? What Sense Motive?

Full Attack. Power Attack.

While using fly to Hover, to get the high ground.

1d20 + 14 = 16, Fly check passes.

Gubbirk has an AC of 23.
Dergaix gets a +1 bonus from being On Higher Ground (Flying).

Bite: 1d20 + 11 + 1 - 2 = 23. Hit.
1d6 + 4 + 4 = 11 damage.

Claw: 1d20 + 11 + 1 - 2 = 13. Miss.

Claw: 1d20 + 11 + 1 - 2 = 26. Hit.
1d4 + 3 + 2 = 8 damage.

Gubbirk has 71 HP.
71 - 19 = 52 remaining.

Sorry, did you say he has higher AC than me?

Well yeah, he's wearing Full Plate.

And higher HP then me?

Full Attack. Power Attack.

Dergaix continues to have AC 19.
Gubbirk has a +1 Greatsword.

Greatsword 1: 1d20 + 12 - 2 = 21. Hit.
1d10 + 14 + 6 = 22 damage.

Greatsword 2: 1d20 + 7 - 2 = 14. Miss.

Dergaix has 59 HP.
59 - 22 = 37 HP remaining.

What the actual fuck.
Is this Goblin stronger than me?

No, your strength is 17 and he has only 16.
Oh you mean in melee? yeah.

You're both CR 6, but he's got worse options in other situations.

It's a fair fight overall, very level appropriate.

If people go through life encountering fair fights every week, on average they will live two weeks. Dergaix is trying to live a few hundred years at least, here.

What did he do wrong exactly?

He assumed that because he met a Goblin he could easily beat, all Goblins he could easily beat?

I'm a Dragon.

He's a little green guy in metal clothes.

So were the Orcs, and they killed your mother.
You're both about the same size?
He has lots of levels in Fighter.

How did he get so many levels in Fighter?

Gubbirk has a long and violent backstory. He spent three harsh years fighting in the Goblinblood Wars, and witnessed many terrible-

Never mind I don't care.

How do I tell how many levels in Fighter a creature has, before fighting it?