I agree he should be getting 3% of hid body weight, but don't just go from 10 pounds to 21 in a day. You need to work him up to it, start by giving him 4 pounds 3 times a day then add .5 to 1 pound each feedind every other day. watch his stool if it starts getting to loose back off on the feed and weight a day or 2. If you are going to switch feed you will also want to do this in stages also. start 2/1 old feed to new then 1/1 then 1/2 and finally all new feed. Make sure he has plenty of good grass hay at all times, helps keep the rumen working.
Good luck
Good luck