I agree you can't force it, but the interesting stuff at that age for me was the fast pointy military aircraft. Wouldn't look twice at an airliner - with the exception of concorde, and GA was just boring. It needed to be loud, fast with flames coming out the back.
Took my nephew to Waddington Airshow a couple of years ago. When the reds came on all he could do was cheer and clap! Other than that the Typhoon, harrier and Tonka is what held his attention, nothing else interested him.
Don't push it but have a family day out at a good airshow. Lots of people on here can recommend some of the better ones. Lot's to do, a lot of fun and aviation themed. Try and get to one where the Vulcan is going to fly again!!