I agree with Ryle. It is not a jab at you personally, but if you can't afford to provide enough forage/hay (2-3% of body weight so for an 850 lb horse that is 17-25lbs of hay per day) and you're worried about the cost of a vet exam then you have no business taking in this new stud colt.
If he does have a health problem, it is going to cost hundreds of dollars likely and no matter what the cause of the weight loss, you're going to be spending even more to get him back up to weight. Regardless of the reason you have this horse (rescue, family couldn't keep him, couldn't resist him) it sounds like you're not in a position to take proper care of him. If I am reading your posts wrong then all apologies and get him on a higher forage diet and a visit with a vet.