I congratulate you on a well-roleplayed dragon.
I think this dragon would now be /very/ wary of humanoids; having just met some who easily could have killed him, I can't see how he would let himself get into another situation like that. Thus, his biggest priority should be to devise tactics which prevent the humans from laying the smack down on him.
I'm sure he also noticed that his breath weapon didn't bother them much. I assume the dragon knows enough of magic to surmise that they are using protection spells. If that is true, I would have the dragon use hit and run tactics to eat away at the group's protections before closing in for the kill. Nighttime passes over their camp would work particularly well (hard to spot a dragon 100' up).
I know you've already given him his spells, but I would give him Invisibility, which would let him really kick the crap out of the party. Turn Invisible, approach, let loose breath weapon, fly away. (It's not cowardice, it's
stalking.)
If you really want to be mean, have it dig a fake lair (using its burrowing, shouldn't take long) between whatever path the party is using and the actual lair. Wait for the party to go in, then fill it up. The party can probably Teleport out, but the dragon doesn't know that, and it should give the PCs a good scare!
I haven't formally joined, but I'm definitely a member of B.A.D.D: in my campaign, a green dragon has a treaty with all the neighboring kingdoms. Anyone passing through the dragon's land must formally acknowledge the dragon's right to rulership, and nothing can be attacked (unless it attacks first, of course). The treaty is centuries old and incredibly complex, making infractions common: and of course, for each infraction the dragon receives compensation. The PCs made the mistake of casting Animal Friendship on an animal in the dragon's territory, which resulted in a most distressing visit to their camp in the middle of the night.