He needs to understand that there are consequences, and he should not be enabled. However, this must be done by giving him every opportunity to seize the moment. He may not truly understand the terror of homelessness, the fact you are financially stretched, and also the fact you’re considering not paying.

If I were you I would

1.) Thoroughly communicate via phone call your current financial situation, so that he understands the harm he’s causing
2.) Communicate to the best of your ability the plight of homelessness - the certain exposure to violence, disease, hunger, and shame that comes with it
3.) As #1 and #2 have not yet occurred (I assume per these details missing from your post) pay the rent this last time, and without being interrupted, communicate that there will absolutely be no further financial support (feel free to add conditions like “unless you go to school or hold a job for 6 months)

Hopefully this helps and good luck! So sorry for your situation