Well I am not sure if I am much help here, But I will give it a shot... I just did a race, actuly my first race in a long time ( got a post on it ) My son was at the track too and race as well in his class. After all he is 8 yrs old. Won his first trophy while I fought a track with new kinds of jumps ( 90 degree typs, oh boy...). But the dirt is in my blood, and could not help it. My 8 yr old walks of with a trophy for first place in his class and never been on a track. All in all, my 5yr old just wants to trail ride, no racing. I ride with both of them just for the fun and admit when I look back and see them fallowing me, can't help but to smile, cause they are having fun with their dad. Like green horn said, bonding. Just because the 5yr old dosen't want to race dosen't mean we can't cruise about in the trails.
so some how try to explain to him what your looking for. and if he says he is too old, BS. I found out at the track I was in the wrong class. I was in 125C and I should have been in the 30's + cause I am 39. And a 50 yr old man who was racing pointed that out. your gonna have to feel him out to find the right way to convince him...good luck, ride on...