I wasn't the one that mentioned the probiotics, but I thought I would mention that I see it in the horse section in my local Walmart, so you might check your Walmart for it, if they carry horse items. It comes in a tube similar to dewormer. It is supposed to give them healthy bacteria for their gut, kind of like yogurt is supposed to do for us.
Where I am at (Arizona), alfalfa is the most common, cheapest hay, and comes in 100 lb. Bales that break off in flakes. I am "guessing" a flake is probably about 6-8 lbs. Of alfalfa.
While a small amount of alfalfa might be good for him, I would still stick mostly with grass hay, maybe with a little alfalfa for good measure. But I wouldn't feed him alfalfa as his sole hay because it is very high in calcium and very low in phosphorous. That's why I am feeding the mare with the foal about 50/50 alfalfa and grass hay (which I guess I forgot to mention in the earlier post). Alfalfa is also very high in protein, which the babies need, and is good for putting weight on them. But if they start growing two fast, they can get orthopedic problems in their legs, so that is why I don't feed it too them straight. That, and the phosphorous/calcium ration is not good in straight alfalfa. So some alfalfa = good, all alfalfa = not so good for growing horses.
I do feed straight alfalfa to my older horses without any issues. It's just the best, most widely available hay we have out here so most people feed it.
If you wanted to feed him some alfalfa, I would get either hay or pellets, because cubes (at least the ones we have here in Arizona) might be too big for him to chew and he could choke on them. I never liked the cubes we have here. I don't know if the are the same throughout the country though. Our are about 2 x 3" and I always were afraid they were a choking hazard.