Hoop De Doo! We took DD when she was 6 and she absolutely loved it. Even though we've been back to WDW several times, we've never gone back to HDDR. However, every trip she mentions it. We are booked to eat there on her surprise birthday trip in March. The HDDR is where we'll be on her actual birthday. She's going to be so shocked! It's fun and the corny humor is perfect for kids around your son's age.
I haven't been to T-Rex, but that's probably a good option if he's into dinosaurs.
Is this your first trip to Disney? If it is, then Chef Mickey's might be a fun birthday dinner for him.