I'd tell your DH that since he thinks it's necessary for your son to be there for the pictures, then he can drive him from the church to your home(1 hour) and then drive back to the reception(1 hour) again. Somehow, I think he will suddenly see the ludicrousness of THAT plan, when it is he who will be spending 2+ hours in the car. I do not think you should put your parents out like that by making them drive your DS back and forth.
I have no problem with an "adult only" wedding. I had one, and it was very nice. Having been to many weddings where there were small children present, I can tell you that I have seen everything from the cake crashing to the ground(BEFORE it was cut)to parents screaming at each other over whose "responsibility" it is to watch the kids at the reception so the other can get drunk and winding up both drunk leaving everyone else at the wedding to watch their kids. I was having none of that! But, I also didn't expect my family members who had children to run them all over Hell and Creation so I could get pictures. My DSIL ended up having her children (ages 5,7,9) come to church and having the babysitter pick them up there, but that was her choice, and she only lived about 5 minutes form church, not one hour.
Tell your DSIL she can't have her cake and eat it too. Either she wants the kid or she doesn't.
You also have the "I'm too pregnant and uncomfortable to go" route, but I always think it's better to tell the truth. As far as maternity formal wear...I'd buy a nice black skirt with a pretty, dressier white blouse, throw on some pearls and call it a day!