You know what, we're doing this wrong. We're coming at it from addressing possible solutions while only defining the problem in dribs and drabs, and no wonder it's not getting anyone anywhere. First step to finding a solution is to properly frame the problem.
The main requirement, as I understand it, is to create a permanent, private space for your stepson to sleep in (and presumably store some quantity of belongings?). Is that right? (Note that I don't say 'bedroom'. Good solutions to this problem might not resemble a traditional bedroom!)
The main constraints are around budget, planning limitations, existing large items like the piano, and (the biggie, really) your partner's veto.
Looked at like that, my immediate thought is to be inspired by like those Japanese hotels - I'd think about creating a little pod for him somewhere in the house, but being creative in terms of shape/size/layout/function. I know this is going to sound a bit Harry Potter, but what's under the stairs at the moment? Could something be put there?