I belive that you can but has your youngster is still growing you'd have to add mineral & vitamin as well as protein such as alfalfa. Personally l've never fed mine oats as they are too hot headed as it is. Someone who does feed them to their youngstock will be able to advise you better
Don't try to force growth as that is the way to create problems with growing; the last thing you need. Steady natural growth (which will be slow due to the breed he is) is what you need without putting too much weight on.
IME something like D & H Suregrow would suit as it has everything in it needed for steady growth without having to add anything at all other than decent hay/haylage and grazing. Ask Adorable Alice on here, she has used it for both Alice and Ted with excellent results. It's very reasonable, a bag would last you something like six weeks and you don't need to add anything to them, for some reason, they might be small pellets but they don't bolt them down, just take their time to eat them. Well worth a look but please don't try to hurry either growth or weight increase, neither are good for growing cob types, they take a long time to mature and by long time, I mean months and years not weeks; there isn't a quick-fix for them unless you want to be causing them problems. http://www.dodsonandhorrell.com/our-feeds/breeding/thoroughbred/mare-stallion/suregrow.html