My 8-year-old's favorite Disney restaurant is Teppanyaki. I would choose some place where you know he'll like the food.
Make sure to note that it is his birthday when you make the ps and also when you check in at the restaurant. Almost all the restaurants will do something to note the birthday, usually a small cake. The most impressive celebration we've had was an almost full-size layer cake, a certificate and a song by all the servers in Norwegian at Akershus. It was my dh's birthday, and he loved it! We ate breakfast on his birthday at 1900 Park Faire, and they brought a beautifully decorated over-size cupcake and had the whole restaurant to sing.
Also, make sure to pick up a "Today is my birthday" button at City Hall in MK or guest services at any of the parks.