He should be fine. The only issue that I can foresee with him being thin is that his back may get sore because it is still bony. You are working on the weight gain and that is good. I would also be sure that you have a really good saddle pad to fill in the gaps.
Well, I'm going to go against everyone else, and say that unless your horse puts on a LOT of weight, and honestly I don't see how that is going to happen if you are going to start riding him frequently, I wouldn't do any major riding for quite some time.
Not knowing how emaciated your horse was, and I am going on the basis that the picture you posted is the most recent one? You don't need to be riding him period. In 3 months, he should be in better shape, and not so thin still.
If the picture is after you have been feeding him for 3 months? Then he must have been pretty bad, and at that point living off muscle tissue, which includes the heart. No way I'd be riding him at all, period, not until he got more weight on him.
Thanks. I have pretty much decided to just wait and see. I have ridden him some, bareback. He hasn't filled out enough for any of my saddles to really fit him properly and I don't think that by Sept. 22 that would have changed. As much as I would like to go, I don't think it will happen. There is another coming up in November and I think the possibility of that one will be more realistic. It will also be a scavenger hunt, so will be a pretty slow pace and not very long. As far as riding, I do hop on him a few days a week but we just walk for the most part. I may let him trot for a few minutes, but he's a trotty horse so it's mostly just to let him get it out. I don't want to rush him...just having some wishful thinking I guess.