Is he good enough for showing?

So you mean I should just take him to a horse show to watch? Hmmm...he gets excited when there's other horses but he's still controllable. I'll take him with a bridle because he's very easy to control then. And then, I'll enter him...if he feels comfortable out there

Yes, at least the first time :) just take him, tie him where he can see the commotion, and maybe take him for a walk around the grounds. Show grounds are a lot of stimulus for any horse, but especially a young one. There are scents, sounds, feelings, and slights that are new to them, all in one. If the show is a two or three day event, you could possibly even take him the first day to watch and compete in a small class the next day!

Being an ex pacer he does have an advantage in that he might already be used to some of this, but I'd still be careful :)

You can always improve his condition if you think he's on the skinny side. As for showing in hand, what have you got to lose. You need to match him to the breed standard or whatever benchmark the judges use. The best advice I could ever give for showing be it your horse, dog cat or anything else is not to take it too seriously. By all means have him look the best he can be but take nothing personally. He may win with one judge and not with another. Above all, enjoy it.