Well. Good luck.
I do agree, however I don't think we can say for certain from that picture that the horse is underweight. He appears it, however he does appear to have a topline that looks worse then it should for his weight. Is it weight? Or damage from severe weight loss? It IS possible that the horse has a muscle disorder like the gelding I mentioned previously (put on weight well overall but still poor topline). So I wouldn't be quick to say he IS underweight personally.The horse is underweight. Having a big belly does not mean the horse is fat. It means that the horse has a lot of hay in his gut.
When a horse (or a human) does not get enough calories, they will turn muscle into glucose for energy. The muscle wasting is most likely the horse's defense against dying of lack of blood glucose.
This is probably not your fault; you bought him in an emaciated state. When the green grass comes out, he will very likely get better.
I would not ride him unless you put tons of padding over that backbone. He will get sores and the riding will cause him pain and therefore he will probably not behave.
I think that the senior feed is a good suggestion.