' I got it massively wrong thinking that on Mothers Day he would just pay £15 for my son to make me happy as part of my treat.'
this is where it all goes wrong
your ' treat ' was only £15 anyway, but now you have doubled the cost of your ' treat ' and turned it into a family day out.
he didn't offer to take you and your son out on Mother's Day, he offered to take you.
Why he did that I have no idea, you are not his mother and you are not the mother of his children, you are the mother of your son but he is not related in any way to your son.
He never met your mum, and it wasn't his child that you miscarried 19 + years ago.
and does he even know you are upset and sad that you won't be having children with him ?
Firstly it's only been 8 months, secondly I guess he must be 40+ if not early 50's - he already has children - adult ones. I wouldn't think that having babies with you ever crossed his mind.
You, as someone has already said, thought you were going to recreate the good old days when your dad paid for you and your mum.
but he is not your dad, this is not his family.
His mistake was to even suggest taking you out on Mother's Day.
did he take you out on Valentines ?