Is he trying to do vegetarian or vegan?

Vegan is going to be pretty damn tough for someone his size.

Vegetarian is more possible to pull off.

Is he doing the vegetarian thing just because he thinks it's healthier? Or is it due to an 'I don't want to eat animals' standpoint?

Assuming it's just because he's got it stuck in his head that it's healthier, and not because of an animal campaign... there are lots of different levels of vegetarianism.

Some vegetarians just won't eat the 'flesh' of an animal. Meaning, no chicken, turkey, or steak. But those same vegetarians tend to not see fish steaks as 'animal flesh' (don't ask me why, I don't get it myself, but that's how a lot of them are). So fish is an option.

Also, some vegetarians feel that dairy products are fine. Which would include whey shakes and eggs. Also things like cottage cheese and yogurts.

So if he's dead set on being 'vegetarian' with things, you can tell him that vegetarians still eat things like fish, eggs, and dairy products. And try to get him to up his protein counts with things like that.

If he's trying to do the full on vegan thing, meaning NO food that came from a living thing, period, that one's gonna be tough. That would mean that essentially all he could eat would be things like grains, beans and veggies.

As far as getting him to eat enough calories, sounds like he's being pretty stubborn about that, and all the harping you do on him about it probably isn't going to do much good. BUT if you can give him some things to read on his own about how going too low on calories is counterproductive to body recomp, then maybe he'll be able to make the decision on his own that he needs to up his cals.