I would personally choose your fav feed company and ask them, I love Dobson and Horrell they are great and fantastic customer care. I explained my mare and what she did whats shes like ect via email with in 3 hours they called me back with more questions and advised me a feed.
At the end of the day theyll know better than anyone
D&H Suregrow is good for youngsters or you could go for baileys lo cal or stud balancer - trick is to get him growing slowly and steadily rather than letting him get fat.
gypsey cobs are usually good doers, so firstly worm well and feed grass and ad lib hay/grass etc. dont force grow a baby on hard feed , not good for his joints. just aim to get him at correct weight with long stem fibre (alpha a and sugar beet) and maybe a balancer. and watch him grow. they grow and bulk up til nearly 6 years. no rush!!