So here is what I suggest:
Get him back into work slowly- lots of trotting and walking up hills will help muscle him up. I don't have hills so personally I do a lot of trotting to build muscle.
As for the feed work him for about 2 weeks and then evaluate him... if he looses weight before 2 weeks is up them obviously please feed him more.
My mare when she wasn't getting worked got 1 big flake of grass in the am and 2 at night... now that she is ridden competitively she gets 2 grass in the am and 1 grass and 1 alfalfa in the pm... she also gets 2.5-3 quarts of Purina Strategy (with flax seed and smart tendon) at night after I ride.
My gelding gets a big flake of alfalfa in the am and if he isn't worked at night he gets either alfalfa or grass and if he is worked he gets a flake of both. He also gets 3 quarts of Strategy (with flax seed and Select 2).
You will have to find what works best for your horse. I would highly recommend that you DON'T give sweet feed or grain. That won't help much. I personally like Strategy(Purina Mills Strategy® Professional Formula GX
) or the LMF products (Welcome to a LMF Feeds
) .. but do your research and see what you think is best for your horse
Let me know if you have more questions.