Hi Sonia!
I know the feeling. I have a 2-year old son, and it's anyone's guess trying to figure who he may be frightened of, and who he won't have issues with. He has no problems with the Chucky Cheese character (although at 1 he did), but he was very scared seeing characters like Tigger and Winnie the Pooh at the park. However, he had no problems with those with human faces, like Aladdin, Disney princesses, etc.
Something that worked somewhat was a few months before our trip, we bought him a plush Mickey Mouse doll. We also watched lots of Mickey Mouse Clubhouse just to sort of visually get him comfortable with the images. Interestingly enough, he had NO problems meeting Mickey Mouse and in fact, was trying to rush him to our table at a character dining!
So that may work for you. The good news is that outside of a character table service, you have to walk up to them (like at a Meet and Greet), they normally don't just walk up to you (invading his precious space-smile). Even at the parades I'd say they keep a comfortable distance.
Also, in come cases we'd hold him when greeting the characters, and that seemed to go over better than having them stand there alone when they walk up. Of course, if your son is, say, 8, that might not work (smile).
Good Luck! Hopefully he'll be ready to go come August!