Yes, I also believe that the cribbing(wind sucking) and low weight are related and common to the Thoroughbred breed. I have known other horses with the same problems and they are always Thoroughbred! I am pretty sure his teeth are ok, she has had him for about a year and I am pretty sure she would have checked them when she bought him.
His owner recently fenced off a 2nd pasture next to the first one so he could be seperated from the others to reduce stress and being picked on in the field. They are only fed the hay and grass along with water in the field, grain is fed in the stalls. I think the beet pulp is what I have heard recommended before to fatten up a horse, I will suggest that and the other things you mentioned to his owner.
Equine and Shell, you both mention garlic for the flies, I hadn't thought of that before but it makes sense if it works on vampires why not flies too! I will also check out the Skin-so-soft, I know it works on kids so why not horses too!

"I forgot to say about the bug bites, make sure all the sweat is washed off after every ride (or if he gets sweaty out in the field) as the sweat will only attract more bugs and more bites."

Shell, after trail rides and riding lessons on hot days the horses get hosed down with cool water and love it (want to roll too!)

"The bug bites, what colour is he out of interest? Ross is dark bay (almost black) and seems to get bitten far worse than all the other horses. Ross swells right up where he has been bitten, to the point if it is in the saddle/girth area I have to ride bareback or not at all."

He is a dark seal brown, looks black in some light except is a lighter brown around the muzzle and a small white star. The bug bites seem to be the worst in the chest and neck area, I hate to curry over them or even brush over the bumps.

Thank you for the answers both of you. Would you believe that we had a snow day here today and school was cancelled! We will be getting more snow this weekend and most of next week so at this rate I won't get to the barn for a few weeks until all this snow melts and the mud goes away!