Thank you to all who have commented.
Mothers Day is always an ‘odd’ day for me emotionally as I explained in my OP.
Just to clarify, my DS is a full time student and has no money as is financially dependant on me and gets a bursary for food at college.
My BF is well paid, financially secure and usually we do go 50/50 as I feel that is fair. The only other times we’ve not done this has been on birthdays where we each pay the birthday persons meal bill. My BF could easily have afforded the whole bill. That said I’m am not the type of person to take someone for granted I can’t stand that. I have friends who expect their BF to pay all the time! I also don’t like people I feel are tight.
I felt in the original invitation to treat me on Mothers Day my BF was going above and beyond and I did feel he was doing this to ‘cheer me up’ as he knows it’s could be a tough day.
I didn’t see that asking him to invite is Mum (who’s late 70s and said ‘if he wanted to’) was compromising him financially. I would have felt awkward about him taking me out instead of her. That didn’t sit right with me. His Mum is his Mum and should be his priority on Mothers Day.
Then as someone has said on here about my DS, I did ask my BF first if I could invite my DS as obviously I want to spend the day with my child (yes 18 but still my baby). My DS really likes my BF and they get on really well so as nothing was said by him about costs I did think my BF would cover him too… £15
I can see now that I should pay for him as he’s not his child. That said I also can’t help but think if he was doing the same outing with his ex partner and they had all gone out that he would have paid for her, his Mum and all his children. Had that been the case he’d have been paying for 5 people.
What has happened has taken the whole shine off what was supposed to be a completely lovely day.
When I raised the question ‘would you have asked your Mum to pay for herself?’ And is the issue the fact my DS came? He said he would have expected me to pay for his grown up children. I said it is different as they work FT but I also said that for example if it were Fathers Day and if I’d offered to pay in the first place, I’d have just ‘sucked it up’ as I wouldn’t have wanted to still not treat him and cause upset over the bill.
I think once he’s resolved the deposit situation and got the £20 back I’ll ask him to send me half (£30) and going forwards I’ll go out just me and my DS in future for Mothers Day. I hate to say it but I feel like he was being tight especially as it was Mothers Day and it was supposed to be a treat for me. If gone from feeling beyond special to becoming a balance sheet.