My DS was 5 when we first took him to DLR because he was a HUGE Cars fan. We spent most of our time there because he loved it so much. I think we rode Mater's 10 times that day! Getting snacks from the Cozy Cones is fun. They have soft serve, churro bites with dipping sauce, pretzel bites, and popcorn. Back then you could get your popcorn in a Lightening McQueen shaped bucket. I don't know if they have those anymore but they still have the Dinoco sippers that look like oil cans, very cute. They also do the DJ's dance party which is super fun. The car DJ comes out and plays fun songs and gets everyone dancing. It's more fun at night, so something to do after dinner maybe since they do it right at the intersection by Flo's.
If your son is a Star Wars fan there is the Jedi Academy over in DL. You have to sign up in advance. My son was too nervous to do it when he was 5, but it was still fun to sit at Tomorrowland Terrace to watch the show while having lunch or a snack. It's always very dramatic when Darth Vader emerges!