First off ... time for a discussion with his teacher. Maybe give her a heads-up on the phone first, and then come with your son and instead of a lesson, or in lieu of part of a lesson, talk about what is working, and what isn't, and what the obstacles are, and what he's finding discouraging or disappointing, and what he thinks might help ... and brainstorm ideas together. Would it help if he had more autonomy, or more structure, or more support, or some contingencies, or different repertoire, or a trio to play with, or more sight-reading, or less sight-reading, or more performance opportunities .... ? When the three of you think back to times when he was more motivated to practice, what was it that was working well for him then? How could you recreate that now?
Hopefully you, he and his teacher can come up with some adjustments to try, as well as ongoing ways to check in with each other to see whether they're making a difference.
I've always explained to my kids that having lessons is a lot like dropping by someone's place and asking for something to eat. If after being given a nice little bag-lunch-to-go, you proceed to throw it in the garbage as soon as you leave, that's really rude. It's perfectly fine not to be hungry, but in that case don't ask for food. "You're asking for instruction, and she's giving it creatively and sensitively and to the best of her ability, and you're just tossing it in a garbage can outside her door. That's wasteful and disrespectful."
As such, when practicing is negligible to non-existent, I've told my kids I will cancel their lesson. I tell them that as much as I hate to throw money away on lessons they aren't getting, I'd rather they disrespected our family's financial resources than their teacher's expertise. I ask them what they think would be a minimum reasonable amount of practicing to do to prepare for the next lesson. Whatever they say, I accept that and explain that I think we should use that as the threshold for cancellation.
If kids are relieved to miss their lessons, that's a sign that they shouldn't be studying, imo. My now-13-year-old ended up taking a break from lessons two years ago by choice after a period of negligible practicing motivation. For the first few weeks she was relieved. Then she started to miss her lessons. So she went back by her own choice about four months later, and while she's not a great practicer, she's reasonably consistent and entirely self-motivated.
Miranda