Thanks @NoviceVillager
Does that ring any bells in your situation? That the WhatsApp was a meltdown message that helped him express an unstable self and now he wants you to treat it as that, rather than an actual accusation?
How I'm reading it is this. Stepson (SS) has expressed some regret to his father that he sent the message. But the regret is more linked to a sense that being under pressure led himself to express justifiable thoughts - that he'd previously been showing manly forbearance by suppressing. And then rather than just accepting what he's said as true, his Dad has put him under further pressure by questioning how solidly based his recollections are and by saying he should contact me and sort things out.
Six months down the line since SS blew up and eight months after the birth of his child, he still feels he is way to busy to be expected to communicate with me. (Freelancing very part-time from home. Partner still on maternity leave.
I think his wiring/temperament means that he cannot listen to alternative perspectives on past events - though he clearly does have some wish to explain to his father exactly why he is right and other people (me and his own mother - it tends to be older women, for some inexplicable reason) are wrong. Because that way we would then apologise to him, for having forced him to tell the truth, and having caused him so much suffering in the past. We would then start to behave again. I would cook him large meals, do the washing up, listen to him talking about his freelance work, ask him questions - but not of an intrusive kind - and admire his baby. But not in a way, that involved my actually saying I wanted to make arrangements to see the baby , because that would be me putting him under intolerable pressure.
I do actually think my stepson has a number of really good qualities, and I've generally - especially in recent years - managed to tolerate/work round the less easy aspects of his character. I have been around him for thirty years. An awful lot has been shared and when I think of 'family', I think about him.
It's something about his having become a Dad and wanting us to be involved with his child - but at the same time to completely back off because he's so very very very mch under presure - that has thrown up the sort of contradictions you were talking about in relation to your own husband.