At 8 weeks, he no longer needs the milk and should be switched over to hay and grain. Instead of the milk, you can add a milk replacement pellet (Start to Finish) at 1# per day. One bag will more than enough. Start him off with 1/2# grain that's formulated for foals 2-3 times a day. Give him an hour to clean it up. If he can clean it up, increase it. He should be able to eat #2 a day plus all the hay he wants (only 2-3# typically). The hay needs to be tender and immature. Easier for him to chew and digest. Get him out of the pen! He needs another horse to teach him some manners and socialize with. He needs to run and play to build strong bones. Identify one of your horses that will put up with a foal, is level headed, has boundaries but won't hurt him. Pen them up next to each other for a few days, than with a halter and leadrope on the older horse, have them check each other out. If nothing exciting happens, let them out together. Foals will spend alot of their time out sleeping. If the other horse doesn't let him, than you need to limit his out time initially so he gets his naps in.