Hi Julee, A first visit AND birthday trip in one...how exciting!!
Enjoying character meals are favorite traditions for our family. The interaction and quality time with the various Disney characters is so wonderful. When choosing your restaurant, please keep in mind that characters are always subject to change.
For a 5th birthday dinner celebration, I would recommend Chef Mickey's. This is a popular restaurant, so be sure to book as early as possible (180 days prior to your arrival). They generally feature Mickey and Minnie Mouse, Pluto, Donald Duck and Goofy. Chef Mickey's has a great buffet, and make-your-own-sundae bar...a big hit with our 5 year old! There are a lot of "kid friendly" options for dinner.
Our favorite character breakfast is the Crystal Palace inside the Magic Kingdom. This restaurant generally features Pooh, Tigger, Eeyore and Piglet. The food is delicious, also with many "kid friendly" options. The restaurant itself is very beautiful, the atmosphere is fun and the character interaction is great.
I think your son would be thrilled with either of these choices. If you start the day at Crystal Palace with an early reservation, you will already be in the Magic Kingdom which may be the perfect Park choice for his birthday celebration. A Chef Mickey's dinner is a great ending to any day. It is accessible via the monorail, which most kids (and adults) thoroughly enjoy. Be sure to tell the Cast Member that you are celebrating a birthday when you make your reservation.
Hope this helps! Be sure to stop back by the Moms Panel with any other questions you may have.