He's currently on LMF Senior. I want to switch it to something for younger horses (he's only eight) that has more protein where he can build up muscle.

This his work out routine:

He jumps twice a week (around six-eight jumps; one-two feet high, in thirty minutes of lunging), then works low two-three times a week (around twenty-thirty minutes). Then he gets two days of just grooming and ground work.

What would be the best grain to switch to with that work out?

(He also gets three flakes of oat hay a day. I'll probably be switching him to alfalfa or grass hay soon.)