im having the same probem, you need too buy a fatty food such as sugar beet or speedi-beat, calm+condition works too. But becareful they do not give him too much energy.
Also mollichaff showshine is great because its helps condition.
but then you need too build up his muscles, schoolings the best way x

I would start with a 5 day course of panacur guard, even if you've already wormed him.  I'm saying this because when I got my warmblood in a September I gave him the 5 day course, and when I gave him it again in the February there were loads of red worms!  I must have missed the larvae stage, and he had been terrible to put weight on, so that was the reason why.  Also get his teeth checked too.  I gave mine alfa a oil and Allen and Page Calm and condition and sugar beet, with flaked barley for when he was jumping and it all worked a treat.