If he has these spots and they are where the saddle is, they are probably saddle sores. Also, if he is thin,he really shouldn't be ridden as he hasn't enough strength or muscle to handle this job. As for his weight, As a general guideline, the total weight of feed per day for adult horses should be between 1.5% and 3.0% of your horse's body weight (depending on activity level). I usually stick around 2% with most being hay, only giving 3 quarts grain twice per day to my most active, heaviest warmblood show horse. My more sedentary retired horse gets only one quart grain twice per day. The rest of the % is hay. If your horses have not seen the vet in a while, I would recommend having them looked over and also having their teeth checked/floated as this could also be a cause of weight loss because they cannot chew and process their feed properly. Advice on feeding should also be sought out from the vet. As well, a good worming program will keep the horses in good condition. You want to be feeding the horses;not the worms! Keep in touch and good luck! By the way, in general, TB horses are "hard keepers", they have a high metabolism andhen require more nutrution, not just when they are racing either.
Has be been checked for worms (internal)? For the bald patches, is he housed with birds? I sent a mare out for breeding and their were peacocks all over the place and she came back with a parasite from them. Hope you get him up and running!!