I haven't tried it, but I assume if it's called Grow Colt, it's made for growing horses.
The way I look at it, now is the most important time to give him a vitamin/mineral supplement. If not now while he's actually growing, why bother later? I don't give anything to my adult horses (supplement wise) but I at least make sure the baby has a vitamin/mineral supplement and try to give him the most balanced hay I can find.