Yeah I'll have to agree he looks much healthier and happier in the top pic, although it does look like he's lost a bit of muscle mass (could be the lighting of the pic though).
Conformation: He's got a cute head and a great soft expression. He has a nice clean throat latch and as tempest pointed out a long skinny neck. I don't think it's disproportionate to his body though, I just think the angle he's got it at is making it look a little awkward. It looks just fine in the bottom pick where his head's a bit lower. Besides it's much better to have long neck than a short one as it helps the horse to balance and move correctly. Moving on, his shoulder is steep which I imagine makes his jog a little rough. It also makes his back a little bit long. He's got a short croup compared to the rest of him which kind of throws his balance off, but I think if you can get a bit more muscle put on him it would help to even things out. His front legs look nice and straight, and he's got good sturdy short pasterns. It's kind of hard to tell about the back legs since he's not standing square, but over all he looks like a nice horse. He definitely needs to beef up a bit if you're going to do halter classes with him, but I you shouldn't have any trouble with the performance stuff.