If you want him to gain more weight, I suggest more hay. 3 flakes a day just isn't enough. I'd be more worried about colic, feeding lots of pelleted feed, and staple feed, with not enough hay.
If you're worried about him getting a hay belly, I suggest working him properly. He should be fine, with the proper exercise. You can't go wrong feeding plenty of hay.
Also, Do you know what cutting of hay it is? First cutting isn't really that great in quality as 2nd cutting, and 3rd cutting has more sugars. If you wanted to put weight on him. I definitely suggest a 3rd cutting, of mixed grass hay..... Second cutting would also be great.
And he won't be as "hot" as with the alfalfa.