How often are you feeding him?
Does he ever refuse meals (other than rats)?
I assume you're feeding f/t? have you tried thawing a rat pup in with a mouse, so it picks up mouse odor? or rubbing one on a mouse?
Have you tried chain-feeding? -first a small adult mouse, & then a rat pup immediately after while he's still in "feed mode"? (make sure the 2 items together
are not bigger than his biggest solo-mouse meals)
BTW, it's appropriate to feed adult male BPs on small rats, not mediums, so at 10 mos. & at his weight yours should NOT be eating either one yet, IMO.

(is this chart showing up for you?)
Young snakes keep growing longer so it takes quite a while before they actually fill "out"....pics would help us evaluate him, maybe your expectations are too high?
Another thing that causes a snake to fail to gain weight is if they have GI parasites (essentially sharing their meals). Have you ever had a vet test a fresh stool
sample of his for worms? Might want to do that, just to make sure.