so, behavior modification. punishment for misdeeds.
i can't remember your situation, but if you usually give presents and he will be getting you one and you get him nothing, then expect negative feelings/resentments from him and possible negative consequences. i think you guys have a kid together, right? if you are celebrating christmas together with kiddo and not getting him a gift will obviously create a scene, i strongly question you to think about this.
if you are divorcing, splitting up, then i would not get him a gift.