Sphinxter What do your parents think of all this palarver? How did they get on with the freakish pair anyways?
Is this the FOG often referred to on here, re your husband being their puppet and believing they´ll wake up one day and be normal? 
I sympathise with your predicament, but you are not powerless. Tough shit if they´re his parents, your son is yours also! For me, I would be wading in there to protect my child from their toxicity ( as Atilla rightly said ), seeing as you´ve got a duty of care towards your own child and everything..... But that´s just me and everyone will handle this situation differently I appreciate.
You´ve got a cooler head on your shoulders than me, and more tolerance too, I´ll give you that! Cos I´d be shouting the odds with my OH if it were me and insist on going full NC with regards to you and your son. Your OH can do what he wants....as long as that doesn´t involve inviting them over obv! 
They need to be made aware that by behaving like this they´re shooting themselves in the foot cos you will no longer tolerate and sit idly by and allow these cringe-worthy Skype sessions. They need to know there´s consequences to out of order behaviour. But cos they´re seeing your son you´re showing them there are no consequences!
However, as somebody mentioned above, a reasonable compromise would be to send the odd picture/vid of your son, but remain NC yourselves. I think this would be more effective than sending them an email, probably falling on deaf ears anyway if it´s already been said. If they see pics of your son, I would think it likely that they reconsider their proposterous behaviour in order to have an actual relationship with him.
However ( again ) would that really make a difference now that you´ve both seen their true colours, what their views are and what they´re capable of? But, honestly, they have no rights to see your son and you both have no obligation to show him to them.
They truly do sound like some glaiket buggers from the Wicker Man or a David Lynch movie. Creepy as fook!
Wouldn´t want these toxic turds in mine or my child´s life personally. You owe them precisely nothing.