Oh come on math only on MN could taking your eight year old son out for a birthday treat be interpreted as some sort of "high standard" it's not. It is simply what any decent loving parent does for their child unless circumstances dictate it is impossible, and in this case they don't.
Not sure about MN, but the OP herself recounted her expenditure and the scale of her birthday treats as if she needed some sort of applause.
This child will be having a birthday treat that most children would appreciate (pizza, friends) with the OP a few days before his actual birthday anyway. She seems to be encouraging an attitude of entitlement in him. What child needs two birthday treats?
In any case, it really is up to the dad to do a birthday that he deems fit given that this year he is the one organising it. The OP has no right to feel put out that he won't be doing something she would do. They no longer live together and it does children no harm to accept that parents need not necessarily be on the same page or marching in unison in their approach to parenting as long as neither parent is abusive, and the dad here is not abusive as far as I can tell. Even when parents are living under the same roof they can and often do do things differently when they are in charge. It does children no harm to learn to live with that.
The dad has a right to do as he pleases when the DS is with him just as the OP has the right to do whatever birthday thing she wants or can afford or feels up to when the DS is with her.
Just as general advice as for feeling pushed aside an 8 year old can be roped in to help with a baby and can be made to feel important and appreciated in his role as older brother that way. It's not always what is done for a child or given to him that makes him feel loved and appreciated and valued. Giving the child the opportunity to contribute is a precious gift.
At age 8 it is important to start getting a child involved in contributing to the household in a meaningful way. Nothing helps an 8 year old older sibling bond with a new baby like being trusted with some aspect of the baby's care occasionally. He could also have some household chores. Measuring the relationship with the child in terms of what each parent gives in material terms to him will end in an attitude of entitlement on the part of the DS and regretful tears for the parents.
At 8 a child really needs to get started on putting the good of the family unit above what he himself may want. It's an important stage in developing maturity. Promising material things that he doesn't deliver is not a good thing, but it is up to the dad to try to explain this to the DS and up to the DS to wrap his head around it. It may well be that the DS will understand that this year's circumstances are different and he may well have enough maturity to get on with it regardless.
It is what contributes to the well-being of the child that counts in the long run and parents doing more than they feel comfortable with or able to provide materially is not going to do that. The quality of the relationship comes from parents investing the time it takes to train children to function well in a group, both in practical and emotional terms and this will pay the most dividends for all concerned. The quality of the relationship should never be measured in terms of what each parent contributes materially.