Ask him! He's old enough to articulate to you what he likes, wants, etc. Sometimes for us it's as simple as an ice cream cone, other times we've gone to the book store after a great report card, picked out a new book for doing a great job, etc.
Maybe your son would like for you to play with him? A game of basketball, catch and lunch at the park, fishing? Other ideas that are "gender neutral" going to the local pottery place and painting something together...I know this sounds kind of girlie, but they have lots of stuff and I've been surprise what the boys have done.
Since it's almost summer, check out Oaklee's Guide for ideas and events. I know my kids love all the local festivals, zoos, etc.
Most of all, I think just getting some alone time is important to talk and bond. Sometimes it's as simple as shopping for school shoes, bathing suits, etc.
Finally, consider joining a YMCA and letting the kids each have an activity that doesn't involve their sibling. Maybe he does swimming and she does gymnastics? It can be same day and same time. But it really helps the kids to have their own interests and friends without having the sibling or twin around.