I don't have any exact knowledge concerning premies as such, but the reason they tell you not to move your child on to cows milk
to drink is because its not nutritionally complete them, and before the age of one, milk should still be the main and most important component of your childs diet...
Formula (unless soya milk) comes from cows.. its cows milk.. they only difference is the processing so that its pure of bacteria and the added amounts of chemicals/vits/minerals. So technically they can have cows milk from birth.. just they will become ill through malnutrition and possible stomach bugs if its the ordinary. Babies can have ordinary cows milk in their food from 6 months because the gut is mature enough to handle it, so even with your LO's corrected age he can currently have cows milk.
However there won't be any issues about giving him cows milk as a drink after a year ... you just need to make sure that nutritionally his diet is complete or you give him extra vit drops.
Personally if I had a premie, knowing that they are more suseptable to illnesses, I would keep your LO on formula until he's two, just because he needs all the extra vits and minerals he can cram in.
But you can use it in his food etc without worry