Biological Mother

Hey, first time posting😊.

I would like to know what are your views on biological mother and father going for supper and movies on child's birthday.

Here's the story:

So I've been on n off with my partner for 5 years, he has a 8 year old son from his previous relationship and I have a 8 year old daughter aswell. Long story short most of the time when we were off he would be with this women n few others. I always felt like he would do anything for her especially if she gave him the light of day. Anyway last year I fell pregnant and he was/is over the moon and really started showing more commitment to me and wanting us to be a family. Then it was his sons birthday and the son decided ( he would do anything for his son) that he wants to go out to the mall for the day to eat n catch a movie n that he wanted his father with. So his father left me and my daughter at home and went out for the day, we fought but he didn't seem to bothered. Anyway fast track I have a beautiful baby n with the sons birthday coming up this is bothering me and I don't want to come off as insecure but I just would like to know if it's right that they go out. Am I overreacting

Btw his son stays with him.. it's always been that way

**Update**

I would also just like to mention that prior to last year the mom had a bf and when it came to the sons birthday they had separate parties and my daughter was involved in the dad's 1 he threw... I'm not sure how to reply to comments anonymously 😊 (he stays with us so and he has a strong bond with his son)