I don't think you can suddenly change the "rules" and have that work very well. Why not get together with your son, make up a "budget" with/for him and try that?

One of my stepsons got in trouble, was spending money for school for other things, bouncing checks, owing money all over the place and got pretty messed up (and didn't/doesn't have a mental illness) financially so my husband did a: "give me all your money" and every week or however often his son got paid he'd come over and they'd look at the budget and my husband and he would pay the bills according to what could be afforded/was due. All that was owed was "known" so he had to get bills out from under his car seat, etc. :-) and he learned about priorities. He'd bounced a $10 check to the grocery store but was wanted in three states by state police for speeding tickets! He was worried about the grocery store, LOL.

Think back to how you learned about money and work life? Like me, you probably weren't ever "taught" anything about it? You have to practice with responsibility in order to learn how it works, just like anything else. We teach math and economics in school but not how to budget and pay bills, use credit, etc. It might be embarrassing for your son and you might want to find a successful male he admires to help? But anything you can teach is probably better than just giving ultimatums without any "help?"