We plan on going the first day...because that is his birthday. I'm hoping because it's not like a meet and greet and he is occupied with food and sitting he will be okay. So if they notice he is frightened will they be a little less aggressive?
.I think a character meal is a perfect idea and Goofy's Kitchen is one of my favorites. The characters are very good with shy or frightened children. They kind of take their lead from the child and parents. I've seen them come and stand back and wave and repeat the same thing each time they walk by the table. If your child is afraid, you can still get pictures of the rest of your party with the characters. At Goofy's Kitchen you get a picture with Goofy before you are seated. That will be your first interaction. You will then know and can alert the other character's prior to them approaching your child. If you are worried, you can alway go a little early and sit and watch other families and children and talk about what is going on in a very excited way to get him prepared.
One of my sons is freaked out about some of the characters, and we took him to Goofy's Kitchen multiple times. He wasn't afraid of the princesses (he LOVED THEM) or any female characters for some reason, but ones like Goofy, Rafiki, Baloo, Chip and Dale, Donald, or any of those he would be scared.
Will your son be scared? Only if he's the one who sees the bill. ;-)