Where to start?
Your son has made it very clear that your dog is not to be at the wedding, so why are you even contemplating it? This has nothing to do with being a dog lover, has it? If you loved your dog you wouldn't inflict a wedding (long journey and a noisy day enclosed with unpredictable strangers) on him.
It seems to me that this is about power.
Your son and fiancee don't want your dog at their wedding. You don't like being told their wishes trump yours, so you're considering sneaking him in, 'just for photos' - or as I see it, for permanent proof that you got your own way.
*obviously I know that he can’t come to the reception* And where would your dog be during the reception then? In the car for hours, alone? Or would you then be deciding to sneak him in after the meal, just as you did to the photos, because it's cruel to leave him out there? You being such a dog-lover?
"The whole thing feels mean anyway"
And there it is. Your reaction to being told that you are not the most important person at their wedding.
"Would welcome advice , thank you"
Clearly not.