He’s so little he won’t remember anything - in years to come all he’ll know of the day will be what’s in the photos.
I totally get your instinct to protect your boy from anything that even slightly whiffs of being used like an accessory rather than treated as a whole person and I would be a lot more hesitant to agree to this if he were 6/7 or so but as a preschooler they won’t be able to force him into doing anything he doesn’t want to do (only bribe him!) and the long term benefit of being in the photos will outweigh the short term negative of being taken back to mum before the wedding party gets going.
I’m sure your boy will be well pleased to get home to you and that being with mum will be a better experience for him than being shushed through a boring ceremony and borderline ignored at an adult celebration.
Pick your battles, you have a lifetime of coparenting with your ex ahead of you and it will be easier for you if you are on friendly terms with your boy’s auntie/nana/grandad etc.
(I split with my son’s dad when my boy was just a baby so I really do get how it feels to coparent such a small child with an ex, my boy is 22 now and well worth all the effort of dealing with my ‘out laws’ for two decades!)
It’s important to find your boundaries so that you don’t become a doormat, forever bending over backwards for the ex but at the same time there is no point erecting unnecessary barriers that make YOUR life harder in the long term and that divide your son’s emotions in a damaging way.
if dad turns out to be a bit useless (and I hope he doesn’t!) your son will naturally learn about that uselessness for himself as he grows and becomes mature enough to understand it as a dad problem, not a son or mum problem.
on this occasion, as long as dad/dad’s family do all the legwork, let it happen. Maybe you’ll have time for a coffee with a mate/a hair cut/a swim/a book or whatever it is that you’ve been missing out on? Even just a chance for a supermarket run without a toddler trying to make you buy things via pesterpower might be worthwhile!