@confusedpasty
We are going to meet up on Wednesday so he can see son and when he is in bed, to discuss the options.
TBH I wouldn't meet him in the house. I'd insist on him seeing DS in a public place, like a park, where you can speak quietly as DS plays yet he will have to monitor his responses and behaviour. At home, it's likely you'll be subjected to a full blown emotional attack. You don't need that.
If that doesn't work I'd arrange for someone else to be there when he's visiting with DS and then stays for DS whilst the two of you speak elsewhere, out of the house. Again, so he has to monitor his behaviour.
Also, at this point, there is no need for him to be able to walk in the house without knocking. You may want to consider either changing the locks, putting a bolt on the doors, or leaving your key in the lock. Changing locks is 'iffy' for a jointly owned home. Personally I'm in the 'better to ask for forgiveness than permission' camp, but you judge for yourself. Bottom line is, you don't need to be home and calm and have him walk in the door unannounced.
I really hope he will agree annulment is the best way forward. It is so hurtful to even consider but I cannot see any other way forward.
Remember that he doesn't need to 'agree' to either a divorce or an annulment. Divorce is 'no fault' and provided you have proof, an annulment can't be contested either. Bottom line, he can't actually stop you from getting either.
Hopefully I can get some proper legal advice before then.
This is absolutely imperative. You need to hear from a solicitor that he won't be able to stop you, which is faster/more to your advantageous. And what you're looking at as far as any financial issues and a timeframe. And if you feel he's going to be 'obstreperous' as far as financial issues in order to delay proceedings, then ask the solicitor about 'bifurcating' a divorce. You can't bifurcate an annulment.
Knowledge IS power in your case. Because once you find out that he won't be able to stop you, then there's really no need for a discussion, is there? And why put yourself through you begging him to cooperate and him throwing every emotional blackmail there is at you. Instead it will be "I have taken legal advice and am filing for divorce/annulment. You will be hearing from my solicitor".
As far as his mother goes, sure she wants you to take him back. Because he and his 'transition' have just landed on her doorstep and she doesn't want to have to deal with it.
Listen, at this point I'd block them both from your phone and email. Set up a separate new email for him to use and tell him it's to communicate about DS only. It serves two purposes; it stops constant harangues because you'll only see his emails when you choose to rather than have your phone pinging constantly. Also, if he emails anything about his transition, you'll have the proof you need for an annulment.