What you're asking is complicated and you should get specific help about your situation. This is very complicated.
In general, if you're co-signing for a loan, then that is not a federal loan and hence not subject to PSLF or anything else. But, there is an exception which is almost certainly what you're talking about -- graduate PLUS and Parent PLUS loans. In a graduate PLUS loan your son would be the primary owner of the debt, and you would "endorse" it -- which is basically the same as cosigning. In a Parent PLUS loan, the loan is completely yours.
Both are very tricky. Repayment options are much more limited than in other federal loan programs. In fact, neither of these programs natively has any repayment options / PSLF (other than just paying them back with fees and interest at the usual rate) -- the way to get PSLF is by consolidating these loans after he has graduated, and that new federal consolidated loan should qualify for PSLF. This definitely can work with Parent PLUS loans. With Grad PLUS loans it's more complicated as it's unclear whose PSLF payments will "count".
If you endorse his Grad PLUS loans, you are responsible for the loan and any fees / penalties / etc -- so if he stops paying and doesn't tell you about it, you'll be financially (and credit report) on the hook for it.
There are many potholes here. If he doesn't complete medical school (? is this a Carribean or other non-US school) you'll have all that debt with nothing to show for it. If you do this, you'll be unable to leave your job (or retire) regardless of circumstances. If the gov't changes their loan policy and doesn't make these loans able to be consolidated, then you might not qualify for PSLF at all (although perhaps that option is baked into the loan documents to start with). There is a ton of paperwork for PSLF, and it's very easy to mess it up. You should make 150% certain your current employment would count for PLSF. Even if so, you'll still have the minimum payments to make, every month, for 10 years.
On the other hand, if your son becomes a physician in the US, they should be able to pay off these loans. If it's a Parent PLUS loan, you'd simply need to trust that they would do so, nothing would stop them from just leaving you with the balance. I guess that could be a financial strategy - they pay your minimum payment to get the rest forgiven in PSLF. There's some chance that would be considered fraud, and if discovered could be catastrophic.
Bottom line - this is nothing to mess around with unless you're 150% certain you understand the consequences. Get someone to help you with this. NOT someone from the school. Someone independent who can show you the problems. There are often free community resources to help with these types of decisions.
