Hi Pinetrails.

I should let you know up front I'm not an expert, therapist or professional of any sort. Nor have I had SM. So, my ideas are just my own thoughts.
First, I'd consider banning the subject of mutism in the house. Full stop. And your husband needs to be on board with this too. Does he want to be "right" or be a good dad? Questioning [nagging, threatening, bribing] hasn't worked for the last 10 years. It's not suddenly going to produce a miracle now. Stopping it probably won't either, but at least it gives your son room to breathe.
Second, I think you're right on target thinking the best opportunity for speech might be as part of something you all enjoy. It might be better if it's not something new. Is there anything you've done on vacations that might be expanded on? Cards or board games [that don't require speech to work]? Computer games, of course. Sports or hiking or favourite TV shows? Museums or exhibitions? Art? Perhaps even DIY [an opportunity to do something creative and practical.]
An alternative to fun things would be something educational. Given that he's truanting, is there a subject that he enjoys that you could get deeper into with him? A little like home-schooling but without withdrawing him from regular school. There are many options online for just about any subject area. If you're learning with him, that puts you on the same level [he's not risking being "talked down to" in the subject.] If it's something like programming, he might even need to explain things to you [that would be an awesome outcome!]
What I'd suggest looking for is ways to spend time with him without needing any speech. For you and your husband to talk normally as part of the activity, but to put no pressure on your son to join in with that part. Building a solid relationship between you, for the possibility that he will never speak again. I hope that doesn't happen. I hope he finds good reasons to rejoin the world of speech. But this covers off the "what if". He knows that you'll still love him, still appreciate him, that it's OK to make this choice. If he feels safe in that, he might start to feel safe enough to venture an occasional opinion or thought. He definitely needs to feel safe enough to express himself in writing. It's entirely possible to make his way in the world without speech, in this modern internet world. But without writing, he's locked out. Again though, I would strongly recommend against trying to force this. It seems disrespectful to me as he's old enough to make this choice for himself. If you had a suspicion that he wanted to communicate but was being blocked, that would be different. It doesn't seem like that from what you wrote.
I don't think there was ever a one-shot deal at "fixing him". The time hasn't passed. But it is moving to being his choice. Whatever his reasons, it's not likely to be attention. And I wonder if it's the opposite. Wanting to "disappear" could be in line with mutism and also avoiding food. And giving him positive attention is not "reinforcing bad behaviour" in my view. Have you had family therapy? Perhaps there's an interaction pattern causing problems? Was there a trigger when he was younger, any traumatic event within a year of him stopping speaking?
Good luck with this! Keep posting, if it's helpful. I would like to know how things go, I hope they improve.