Hi dafpri, great question about boundaries. I think you're on the right track with thinking of boundaries as something that could help you when you're in this place of feeling extended past your limits.
One aspect of boundaries that is important to consider is that they are different from requests, desires, or demands. Boundaries are rules we have for our own lives, that we are 100% in control of. A true boundary doesn't require anyone else to agree, cooperate, understand it, or think it is a good idea.
I bet we can reframe your desires for your son's behavior into boundaries for yourself
For example:
Desire: I want my son to watch his spending
As good as a desire as that is, because you cannot control his actions, it isn't a boundary.
We can convert it into one:
Boundary: I choose to give my son $200 per month. If he runs out of money, I respect his choice, and I do not rescue him. I choose to wait to give him more until the next month.
Notice how that is fully under your control?
I wonder if you can translate your other desires for him into rules for yourself that you control?