AITA for insisting my son be the flower boy?

I am getting married in July (:D!) and wedding planning is in full swing. My wife to be and I are on the same page about almost everything, except for this one thing. My son wants to be the flower boy, and I want him to be too.

My fiance says that a flower girl is traditional and that it would be weird to have a flower boy. She says her family will be uncomfortable. She says that he should be the ring bearer. He doesn’t want to be the ring bearer, he wants to carry the basket of flower petals. He thinks it looks fun. My line of reasoning is that anything that gets him excited about this wedding and having a stepmom is a huge plus.

My fiance says we shouldn’t teach my son that everything is about him. I absolutely agree. However, it is weird for kids when a parent gets remarried. He hasn’t engaged much with the idea, and this is the first thing relating to the wedding he showed any excitement about. My fiance wants to know what he would wear, and I said the three of us can definitely find something cute. She says she doesn’t want that extra task on her plate. I said then he and I can do it, and she said she would be stressed about not knowing what he’ll be wearing.

We have been going back and forth about this for a while, and she is starting to get frustrated. Yesterday she said she didn’t want to talk about it anymore. I said we have to resolve it, because the wedding is in seven weeks. She said as far as she is concerned, it’s resolved. I said that in that case, it’s resolved for me too, and he’s doing it. She asked me why I’m being such an asshole about this.

I don’t think I’m being an asshole, but maybe I am. I haven’t been pushy about anything else with the wedding. Whenever I wanted something that she didn’t I either compromised or let it go. This is the one thing that really matters to me, and I think it’s ultimately good for both of us because it will make my son more fond of her. Am I really being an asshole?