I'm also going to vote for not riding him. Not until he's feeling healthy again. Being that emaciated would leave him too weak for carrying that extra weight around, in my opinion. Not to mention how uncomfortable he would be with that protruding spine. I mostly fear that if you continued to ride him in this state, that he'd be extremely uncomfortable and in pain, and he would remember that. So when he IS feeling healthy, he will have already connected riding to pain, and likely develop behavioral issues as a result.
As for his diet: Might I suggest beet pulp? That stuff worked wonders for any horse I've seen that needed some weight put on. I would also think hay should be provided; forage is just as important, if not more important, than hard feed. Also, I've never heard of or dealt with 'wasting disease', or EPSM, so I can't advise you there. However, until your vet can diagnose him with this EPSM, I would be treating him like the severely underweight horse he is.
With all that being said, I would like to congratulate you on having the heart to take this poor boy in. He may not be strong now, but it looks like he has a very kind eye in that picture and I'm sure he will be very grateful for your hospitality.