Any Adjustable Rate Mortgage re-setting/casting can be unsettling. The bottom line… Read your Promissory NOTE. Understand when it changes, what caps it has, and figure out what that means to your payment and your budget.
I believe some lenders will allow for a non-occupant co-borrower help qualify, If you are IN CA I can help. Otherwise interview a couple of mortgage brokers in your city to help you understand your current loan, what happens when it changes AND if needed what options/loan programs you might qualify for.
You’re correct that when HELOCs recast the payments are amortized over a shorter period and include both principle and interest, unlike your current interest only payment. There are certainly loan options available that allow non-occupant co-borrowers. Of course, both borrowers must qualify based on their credit, but the incomes (and liabilities) are pooled for loan qualifying. I write loans in all states, glad to discuss your situation if you’d like.
It is possible that a VA loan would be a much better option than a variable rate HELOC. VA backed loans have very good rates and terms.
Another option, if your son is willing, would be to deed the home to him on a life estate. You have the right to live in the home as long as you are alive. He could then take out a loan to pay off your HELOC. You could then make the payments to him if he requires that.