Whiff, I am so sorry - any time would be bad for this sort of upset - but at present it seems so far worse.
Adult children often write their own version of their childhood history. My youngest daughter has done this for years, and has 'memories'', which none of the rest of us share!!!
It was she that I often felt I put too much emphasise on, sometimes to the detriment of her slightly older siblings - yet she has a very strange recollection of those times.
We have had quite serious fallings out over the years due to this.
Now, we maintain an uneasy peace. I now she wants me to be part of the lives of her daughters - so we do not (in her company) ever talk about the past. Her older siblings do laugh about her false memories when she is not present.
She, and her twin sister are very close - and her twin will defend her staunchly no matter what, we have to be careful in her presence also.
Fortunately, everything is a very even keel at present, and will stay that way. As they live on East Sussex I can only see them three or four times in a (normal) year - and when I go there I do not stay at their house - despite them having a spare bedroom - always go to a B&B. Think, an overnight stay might be pushing it a bit far - use the excuse that I can only use a bedroom with an en-suite). So, we are all really good friends at present . Must admit, that very occasionally, I will drop into a phone conversation we are having, some small detail about events that happened in the past -usually try to make these funny anecdotes - but they are actually being said to try to get her memories on the correct path.
Whiff, Give it time with your son - it is a very emotional time for all of us at present. So, he has dredged up memories -which can be totally false, and then put his own twist on them.
Don't push him at present, just let him know you are dreadfully sorry that he feels like that - would love it if he feels able to talk about any of it with you - but in the meantime, let him know how much you will always continue to love him, be there for him and hope he can let you in to have some involvement with his children.
Do not let him and his sister fall out over this - I am sure, that once the new baby is born, and life starts to return back to something like normal - you will find ways of mending this breach.