First of all, I would probably just feed him for at least a month, until he returns back to normal weight. Then after he returns back to normal weight, I would then start to train him in the things you have said you would do, keeping in mind to still feed him a little extra, becuase:
1.) The more you work a horse, the more you need to feed him. Although I'm sure he probably won't be doing THAT much work.
2.) He is probably still recovering, so just keep feeding him a little extra, just to make sure.
To me, health comes before work. So to put it in one sentence. Just get his health up before you start to work, train him etc.
Hope I helped! Please tell me your thoughts.