Your son seems to be very wise. He knows you are the stable factor in his life and it must concern him when he sees you take less than a healthy choice. He may even have thoughts if something happened to make you sick where you couldn't take care of him, then what?
Under other circumstances I might give a different POV, but since you wish you had S12 more, and since conditions at the aunt's are not the best, I would say to be flexible and let him stay with you. I think it would take a little stress from him. He may feel victimized when he's force to stay there. I doubt it is going to make her get her act together any faster by pressing it. In most cases I do not have this opinion regarding scheduling, etc. However, for S12's sake, I say to go with the flow and see how that works. You can always make adjustments later.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!
