If you feed straight alfalfa then your Ca:P ratio will not be balanced. You can balance it by feeding grass hays. It's easy to stay balanced with a diet like grass & Strategy because they make Strategy for a grass diet. They used to make an alfalfa formula, but I don't know if they do anymore.
When I lived in a state where I fed alfalfa, I always fed 50/50 alfalfa/grass hay and I bought straight alfalfa and straight bermuda grass hays. Part of the reason I fed the grass was to also keep the horse busy chewing and eating small meals all day. They tend to eat their alfalfa ration and then have several hours of nothing between meals if you only feed alfalfa. Are you going to have your horse out on pasture during the parts of the day when he's not being fed? If he's not going to be on pasture, I recommend some kind of grass hay to keep is gut healthy.
You may find on the straight alfalfa diet, as he's losing weight now, that you need something like Strategy to hold him at the weight you want him at when he gets there. That's perfect to put the supps in, you'll just kind of have to feel out how much Strategy to feed him when that time comes. Might be 1 pound, might be 3, you'll only be able to tell when that time comes.