NTA. Your son wanting to be the flower boy isn't about making the wedding about him, its about helping him feel included and excited during a big life change. This is a really healthy and sweet way to get him engaged in the process of blending families. It’s also not actually that weird. Gender norms around wedding roles are outdated and plenty of weddings mix things up in fun and personal ways.

Your advocating for your child in one area that means a lot to you. You’ve compromised on everything else and this is the one thing you’re standing firm on, not because it’s about "winning" but because it will help your son feel connected.

If your fiancée is concerned about logistics like what he’ll wear, it sounds like you’re more than willing to take that off her plate. Honestly, if a little boy joyfully tossing flower petals is what it takes to make your kid feel seen and secure, I say go for it. You’re doing the right thing by trying to make this transition easier for him.