“feels appropriate” doesn’t have much bearing on how much horses need The average 1100lb horse in light work needs about 700gm CP, around 30gm lysine, and I forget how much methionine.
Without a forage analysis you don’t know where you are. But you can look at muscle development and see if it seems appropriate for the amount and type of work he’s in. If not, then if you feed only grass/grass hay, it could be a CP deficiency, but might just be an essential AA deficiency, specifically lysine and possibly methionine
If you feed significant alfalfa, it’s unlikely there is a CP or AA deficiency