I am not in that situation yet. Exhusband still has maybe 10 yrs before he considers retiring. I am just trying to see what could possibly happen when that time comes.
Okay.
Regarding your first question: if you have a court order for child support, that court order remains valid no matter how little he sees the child. Perhaps Dad is thinking that if he voluntarily terminates his parental rights, he will be off the hook. However, this is only going to happen if there's a step-parent adoption. You have not mentioned any plans to immediately remarry, and even if you were to marry someone else, you'd have to be married over a year before even considering a step-parent adoption. Oh, and Dad can't force a step-dad to adopt; stepdad has to actually want to adopt.
The obligation to pay child support remains so long as the court order is in effect. Of course, Dad can petition for a downward modification, but unless you agree to it, he has to come up with reasons that are legally valid. This includes
involuntary reduction of income; voluntary reduction of income (he's always wanted to go back to school and become a professional underwater basket weaver) won't do. Significant changes in the parenting plan that result in him having more parenting time could also justify a reduction of child support - if you had 50/50 custody and similar incomes, it would make no sense for him to be paying support to you, for example.
Regarding your second question: if he takes early retirement, that could be viewed as a voluntary reduction of income, as stealth pointed out. If your ex-husband will be standard retirement age, that is another issue. Your son will be 18 then, so not a minor. Dad is on the hook for child support until his kiddo is 19 or graduates from high school, whichever comes first. Dad could be on the hook longer, depending on what the decree says about post-secondary education, and your child's particular circumstances.
I have not addressed alimony, because it is unusual nowadays for alimony to be long term. If this is a case involving VAWA, then that's another story. For the purposes of social security, perhaps someone better versed in that can help you, since you've been married for over 10 years, but in the US only 9 before filing for divorce.