Well the son thing is an easy fix. 'I didn't have an affair. Your father coerced me into a threesome. I just did it to stop his damn asking. And now he's coming out with this shit! Incase you hadn't noticed, he's an arsehole'.
Unless your son is the same as his father, he'll understand. Don't let his dad tell him lies like that and let it slide. Though I suspect he already knows and is just like his dad so is playing along. Or knows but knows if he doesn't pretend to believe his dad, and treat you badly, then his dad will turn on him too.
Tell your son the truth.
A little embarrassment is better than him believing his dad's crap. Add 'i hate it when he lies to you and uses you like a pawn'.
And get away from your psychopath of a husband because he's dangerous and means you serious harm. It doesn't matter what other people believe. It really doesn't. Abusers convince you the whole world is looking and judging. The truth is, most people who are closest to you know he's a pig, even if that are in denial. They also know you probably didn't cheat.
Time to say 'enough' and leave him. Irregardless of if you'd been the perfect person and partner he'd still lie and make put you were bad. The trick is to stop caring about what he says or thinks. Just go 'oh he said that did he? Haha honestly what next?' And roll your eyes, should anyone mention the shit he says.
Get free. Cut out any other toxic people from your life too.
I know it seems like you're in a cage atm. But you actually have a key to it. It's far less scary outside the cage. Go!