Hi freewill,
I do hear your worry. I agree with the rest of the posters that you are not obligated. You do only what you are comfortable doing. That may mean that you simply buy them a gift and wish them all the best. Your son knows your situation and can't possibly expect you to finance their wedding. I would stress to him that you can't give what you don't have. He should be able to understand at his age...

Stay firm!
Okie