FWIW KBoys, my opinion is "absolutely not". There is NO WAY - absolute zero chance that I'm incurring debt to pay a deposit for him. That's a Band-Aid on a broken arm & only buying you time until his next "need" surfaces. Put the money toward the kids.
"No" only has meaning/power when you stick to it - if you roll over & go back on it & turn your no into a soft yes/maybe, you're showing him that if he keeps pushing eventually you WILL cave. (like letting him stay Sat/Sun) We give away your power when we flip-flop like that.
He has 100% of the control & ability & power to change all of this into something else - it's up to HIM, not you. (((((Hugs))))))