He is old enough. I have weaned colts at 4 -5 months of age. If they are eating hay well and a junior feed, I would put the colt with a "babysitter" old pony. Horse are herd animals and hate to be isolated. As he matures, then introduce him to his own stall or paddock- but make it so he can see other horses. The less stressful his life can be, the better he will
grow and learn! ;-)