You should add him to your reservation. He will not have a ticket for magic express or a magic band if you don't. When we went with an under three it was before fpp but we did not need a fp for our daughter.
We just went a couple weeks ago and our 2 1/2 year old was on our reservation and got a magic band but never had to scan it for anything and was able to get on with us with our FPs. Does he have a magic band?
They give magic bands to everyone in the party, but kids under 3 don't need them. My son wouldn't wear his. You don't need to make FP for kids under 3 either. We had two in our party without any issues. CMs never even asked about them.
He can get a magic band, but won't use them. And you can't make FP+ reservations for him because he doesn't have a ticket linked to his band, since he doesn't need one.
Just want to add, kids under 3 are free at buffets (Character Meals) and dinner shows. For other restaurants, the kids can share the adult meals or order their own.