I assume we’re talking about a yeshiva (not an MO HS that charges 20k)- With your financials you can get a yeshiva to reduce tuition to $500 a month. No one is going to give you a contract for $100. Just voice your concern, you showed them your finances already, no doubt, and sign the tuition contract. Try you best to pay it, when you can’t, they have already been forewarned. They won’t kick you out. You’ll deal with next year, next year. It’s going to be tough, somewhat humiliating, but they will let your son stay, you’ll again try to make payments etc…
I don’t see any other solution. This is what many many parents that are struggling are doing.