Poor guy! Putting a 2yo on months of stall rest - must be pretty serious injury - what happened to him?
Is he stunted or some such due to early malnutrition? Why did the vet suggest this? Without knowing why or what specific feed you're talking about, generally speaking, something called a 'colt grower' makes me edgy. Sounds like something that will be high energy, high protein, designed for 'growing up' horses quicker. Hence DODs, not to mention health probs from just too much rich feed come to mind. Especially if he's already too fat. I wonder why the vet would suggest it, aside from feeding an already fat horse more feed, especially when they're on enforced rest?? Makes me wonder about the vet's knowledge on the subject.
Absolutely don't feed grain to a fat horse, particularly if he's still growing. If you need to feed something 'to get the meds in', there are better alternatives to grain & such, and you should also be able to cut down that amount substantially. Beet pulp & copra come to mind. Good idea to put the hay in a slow feeder, as that will hopefully slow him down enough not to gain more weight.
I imagine that 'Grand Complete' is a low dose 'ration balancer'? I think you're on a better track with that kind of thing & Smart Pak does seem like they have some very good products. All he should generally need aside from hay is whatever minerals appropriate to balance his nutrition. As it depends what he's getting from his hay etc, that will affect whether certain 'balancers' are more or less appropriate. I'd also be looking into supplementing extra magnesium, especially as he's already fat, injured & will be under extra stress being locked up.