I'd do one of two things.
1) Confront mom before the gift exchange and gently, politely, say something like, "I appreciate the intent of this gift, but it's undermining what we're doing. It probably would be better for you to give this to someone else. Thanks for understanding that I'm not ungrateful, I just want to give something specific to Son."
2) Take the gift gracefully then give it away, smash it on an amp, whatever. When questioned later as to its whereabouts explain, "well, it wasn't in good shape (didn't meet our needs, etc.) and we found one that suits Son better."