I would suggest that you two take time to talk - actually, you talk and he listen. Make that clear up front, he's had his say, now you have the chance for rebuttal as you haven't weighed in yet, but that you're also going to seek his thoughts in this as it is still a conversation. But make it equally clear that you are not deciding this one way or the other for him - that, I believe, will open up the ears.
He needs to understand what you think will happen if he doesn't stay in school, the possibilities that can occur - both good and bad (don't slant one side too much, it shows your mind is made up - and whether it is or isn't is irrelevent, you still have to be in the game as having an "open" mind). Try to get him to understanding this by asking him questions of what he thinks will happen - and then play out the scenario ("ok, now if that were to occur, then what would you do... "ok, now that you've decided that, these things will follow, then what would you do... etc.). Then keep him engaged.
After making sure he really understands the "real world" ramifications of his decisions, then he needs to know what you are going to do and not do (that is, financial support - which includes place to stay, and so on - this has to do with the fact he really is an adult and that if he wants to make adult decisions there are consequences he needs to consider). This is not so much as to be a hammer (it might have that practical application, but that is secondary), but as knowledge he needs to have to properly formulate a decision.
Now, after all that input and debate, if he still wants time off, and he's willing to do it on his terms, then that's his decision to make.
This is stuff I'm sure he hasn't even thought about.....