How bad are the bugs? In the close up photo I can see the welts. Stomping bugs in the heat will take weight off very quickly and hinder any weight gain.
The last 2 pics look terrible.. Honestly in person he doesn't look that skinny. Even my farrier said he's seen skinnier. But you can tell he is definitely slim. When he first started loosing weight I talked to the vet on the phone and he said to give him a couple months to gain it back on but still nothing. The vet also said it was a TB thing since he has TB in him and they tend to be "lean" but I think he is most certainly underweight! We'll see what the vet says..
As for the teeth I don't know much about it I just know he got it done not that long ago but he really doesn't act like his teeth bother him at all..