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.
Just not worth the risk.
Horses make me a better person.