Hi Hellen!
I would contact 407-WDW-DINE as soon as possible to try to reserve something. You still have time, but each day closer makes it tougher to reserve something you may want.
Chef Mickey's is my recommendation to see Mickey and friends (Donald, Goofy, Minnie, and Pluto), but it is a favorite for guests so you may find slim pickings for reservations here as this is peak season. The restaurant is a busy restaurant, but the seating I'd say gives sufficient space to where we didn't feel we were right on top of the guests next to us. A good alternative to see Mickey is Donald's Safari Breakfast at Tusker House (Animal Kingdom).
The only dining to see Playhouse Disney characters is Hollywood and Vine at Hollywood Studios (where he'll get to see characters such as Disney's Little Einsteins). This restaurant is a little cramped, but your son may not pay much attention as characters are constantly moving about here, and they dance and sing some in the restaurant, which makes it fun and exciting for children. I believe the characters here are only available for breakfast or lunch, something to note when making reservations.
Good luck on your planning and have a magical time!