I certainly wouldn't be feeding a mini competition mix. I would look into suregrow or a balancer to make sure that he is getting all the vits and mins he needs. I personally would feed more hay than concentrates, if you are feeding a vit and min supplement this should be sufficient. You could ring some feed companies help lines these would be happy to help and would be able to tell you which of thier products would be best.

On the other hand my mare always goes a bit 'dull' at this time of year, I give her a course of global herbs 'restore' which really gives her a boost. If he is having a growth spurt ths could take it out of him a bit.