TBH Cobs tend be be good doers even as youngsters, so I would feed mainly fibre like alfalfa which has good calcium and protien levels, with either a balancer, like Lo-cal or Evenkeel both of which have low DE, or a general purpose supplement, you do not want to make him fat, but he does need his vits and mins to ensure even growth. Keep it simple and you should not go wrong - if he gets too fat on alfalfa, change the chop to one with a lower feed value
Hi, I'd suggest a fibre feed like Hi Fi or Hi Fi Light (if he's a very good doer) with something like Dodson & Horrell's Surelimb supplement. It's designed for growing youngstock so will provide everything he needs without over topping him.
Hiya
In my opinion best thing to do is fill in an online request form on a feed companies website, when i was looking for a youngster i found them really helpful, think it was dodson and horrell.