The alfalfa will give him a little extra calcium and protein. He needs it for growth( assuming he'll be a yearling pretty soon). Start him out slow, with just a handful and add more over the course of a couple of weeks. If you get hay, a flake a day should be fine, with pellets, soaked, together with his RB, a pound a meal. Dry weight. The pellets have slightly higher nutrient content than the hay, mainly because they don't waste anything.
I fed my mommas to be, the babies, youngsters, and adults soaked alfalfa, with all necessities, and they were all fine