Obviously, only you know the circumstances of the conversation, but I can't imagine a coach making an iron clad promise to a kid that he'll play a position an entire season regardless of performance or any other circumstance. Seems to me he gave the kid a chance and decided the lineup was stronger configured other ways.
My advice would be (1) take this as input that maybe junior fits better at another position and/or (2) even if position X is where he belongs, experience at other positions can be invaluable particularly down the road as he looks at the next level.
Bottom line from my experience, if you can hit you'll have opportunities to play and if you don't, generally defensive prowess alone won't be enough to keep you in the lineup if there are other options that are hitting.