I think you're being very difficult about this, OP. You have several options - you just don't like any of them, and it does rather seem that you've taken umbrage about not being able to take your baby along, looking lovely in his little suit, for the whole event.
You are clearly able to leave him with your MIL, and she is willing and able to have him, so you already have considerably more flexibility than many people in this situation. You say 'but she's 68' as though that's akin to being 108. My lovely MIL is only a few years younger than that, and is entirely fit and healthy - probably fitter than me at the moment. You're content for her to have your baby during the week, so she's clearly not infirm.
You could take him for the evening, and leave when he gets cranky. There's absolutely no reason why your DH can't stay, except that you don't want him to. He's the best man - not just a random guest. It would be odd and rude for him to disappear early, when he doesn't have to.
You could go to the ceremony and pick him up afterwards. You could go back to your MIL's place latish and stay there, so you're there to do any night-wakings. You could ask her to drop him off with you after the ceremony.
There are various ways forward. If you don't want to take them, then don't. But it would be very churlish indeed to prevent the best man from participating in the entire wedding, simply because your baby isn't invited to the ceremony.