I like everything about this piece except for the fact that you covered up his face with the bow/arrow situation. His clothes just need more ornamentation. Cuffs on the sleeves, buttons or some sort of fastners for his vest, hair braids(?) patterns, etc. That's a big bow haha.
I like how this character is developing. I like his costume so far, but I think you're right that it needs a little more something. At first I was feeling like the horse/human proportions were a little off. Like the human part was a little small, or the horse part too big, but now I think that the horse part of the character is more empty than too big. Maybe you could try inventing costume elements for the horse part to unify the character's two halves and make him all the more interesting. Ideas: He has those cool gloves, could that sort of element be repeated on his legs? Horse bracers? You could repeat that green neckerchief, even in a different color some draped cloth could be cool. Maybe he carries his quiver on his horse body? A satchel strapped on? Not-saddle bags? Does he use/have any other weapons like a sword or knife that could be on there somewhere? It's like I'm wanting to see something like how horses always have lots of cool saddle stuff and leather work and straps and stuff that's all layered and complicated and COOL!! But it'd just have to be thought through to be appropriate for a centaur. My hubby's 2 cents are to remember horse blankets if there's anything heavy that would wear/rub on his back, and he also thought that the horse part was pretty smooth for a powerful horse body and could use more horse specific muscles. Maybe more muscles would be all the horse part would need. I never thought I'd be telling you to put more muscles on a character. :)