How's his look? Should be able to feel last 2 or 3 ribs with your fingers without too much trouble.
If he's starting to look out of shape then back off on the amount your feeding per day by a ¼ cup, feed that for a couple weeks keeping an eye on body shape and weight. If still gaining keep doing this ¼ at a time until you see the weight level off or he visibly looks in better shape. Unless he's got some underlying health issue (which I doubt) it's normally all about calories consumed vs calories burned.
By the way, how much are you feeding per day and what formula are you feeding? Maintenance... Performance etc...