yes, he filled out Fafsa. The tuition is 80K/year (Ivy leage), I am cosigning for 20K per semester, so only for 40K per year
Ummm, this is not an "only." Median household income in the US is somewhere around $70K, while the median personal income is around $45K. See for example
https://worldpopulationreview.com/state-rankings/average-family-income#. You are effectively signing up to spend the US's median personal income every year, just for your son's schooling. And worse: Current You is not paying for it, because Current You doesn't have the money available, so Current You is passing the buck to Future You.
My impression from the posts so far is that you really don't have a lot of information about what you are diving into here. That means that you are agreeing to take on somewhere between $80K-$120K in debt (depending on how many credits transfer) without really understanding the terms of that debt or how it will affect your finances long-term -- or, indeed, investigating other options for paying for your son's education without taking on what is effectively a mortgage on a small house.
Please please do not sign anything without fully understanding the financial impacts on you. Assume that you are 100% responsible for this debt -- because in the lenders' eyes, you are. Assume that the PSLF program goes away. Assume that you cannot get this debt discharged in bankruptcy. If all of that is true, will you be able to afford paying the loans? Unless your answer to that is a resounding "yes, of course," don't do it. Just don't.
Please do talk to someone at the college about what his and your options are. If they need more information from you about your finances to qualify him for more need-based aid, then get that together and get it submitted. If it means he needs to defer admission for a semester or two to hit the next financial aid cycle, that is far, far preferable to crippling yourself financially with debt you cannot afford. You need far more information than any of us here can provide. It's worth taking to the experts -- people at the college, your own financial advisor to help you evaluate how the loan will affect your budget and retirement plans, etc.
And remember: just because your son got into an Ivy doesn't mean he has to go. Yes, it is fantastic that he got admitted, and we all want to give our kids the best start in life. But that doesn't mean we need to -- or should -- pay for our kids to have things that we can't afford to give them. And there is good data suggesting that a fantastic school matters less than the characteristics that allowed the kid to get into that fantastic school in the first place. IOW, if your son is such a great student that he got admitted to an Ivy, he doesn't actually need the Ivy to succeed in life, because those same qualities will allow him to succeed equally well somewhere that doesn't cost you $40K/yr in loans.