When planning a wedding where there’s children from one or both sides involved, I think you have to make sure you see the event as a joining of two families, not just a joining of two people, and everyone’s needs have to be taken into account.
I think you have to remember that whilst yes, as the bride and groom it is your big day, the kids NEEDS always come before your (*your fiancés) WANTS, and that’s just the way it is.
Your son needs to feel involved, or he’ll feel put out, understandably! Especially as there’s no need for him not to be part of the day...
When my husband and I got married, his 13 Y/O daughter was his ‘Best Girl’ - and took on the role of best man, and she was absolutely amazing. Made a cracking speech and held all the ushers to task! His younger daughter was a bridesmaid.
Didn’t matter at all that it wasn’t the ‘traditional’ set up, it worked brilliantly and everyone was happy.
If your fiancé is worried about odd numbers in photos, I’m pretty sure that given the circumstances, if asked, any of your bridesmaids would step down to ensure that your SON could be involved?
Practically, could your son walk you down the aisle or is he confident enough to do a reading?