I think a great way to see how he will react to the characters is by seeing a parade. That way he can see them, but not have to get too close if he gets scared. At MGM they have the stars and motor cars parade which we love. You will see all the traditional Disney characters as well as Playhouse Disney ones, Cars Tow Mater and Ightning McQueen, muppets and more. There are also so many great characters at MGM. You would have to get the guide when you enter to see the times and places for different characters, but we have seen the Monster Inc. characters, Cars, Green Army men from Toy story and so many more. We were able to see Buzz not far from the Buzz ride at MK. If you do a search on the DIS there is a person here who has a website devoted to the characters with pictures and info on where to find them.
At 2 and a half DD was afraid of the characters and wouldn't go near them. She was better with the face characters. DS was always OK with them. When he met Woody and Jesse in May, he ran right up to Woody and gave him a great big hug!!(he was 3 yrs old)