I understand your point about wanting to buy nice things for your child. Something to keep in mind though is that objects aren't the only things that help children grow. It is my understanding that Boise has some interesting attractions that you can visit.
If you can frame it as a vacation and visit some things like the botanical garden and the center for birds of prey, then you are creating memories for your child that are just as important as things you could buy him. When I think back to my childhood, it isn't the Christmas presents that stick out in my mind, it is the vacation to Disney World, or going to a baseball game with my dad.
Also, if you are doing the hermit lifestyle thing (which I totally get) it would be a good thing to expose him to the outside world, and travel is a great way to do that.
My suggestion would be to skip the musical instruments and fund a family vacation that includes time for you to participate in regionals.
If the main issue is your spouse, then let her know that there is help out there for her. Usually you can find free/cheap mental health support, especially if you are below poverty line.Then maybe in a year or two, you traveling by yourself won't be as much of a hardship for her.