Does he get to see other kids riding including balance bikes? Kids follow kids.
+1 to this. My first didn't really show an interest until I built a second one for a friend who'd had a kid at a similar time & was going away. As soon as he saw the other kid on it riding around, he was all over it. My second was into it straight away because he was seeing his brother riding all the time.
On to the original topic though, I recently turned my front yard into a bit of a pump track for the kids, 1 on a balance bike (well, was until last week), and the other on a 16" (now 20"). Berms on the corners, a small (15cm) table top drop & a kicker at the bottom for some jumps. My 2 yr old does his best to launch the kicker to copy his brother & lands pretty hard. That poor bike's days are numbered given how rough he is with it. They love it though, not just for riding but also for getting out & helping me build it. Eventually I'll have to remove it & put some grass down, but I figure I've got until spring at least on that one.
I also take them down to the skills area around me fairly frequently. A lot of teenagers there at the moment, but they mostly stay to one portion of it & leave the other bits alone, which works for my boys.
I'm thinking for the next project I might build some ramps & bridge / log sections for them to practice on out the back. I've got a ton of spare timber lying around, so figure it'll be good to use it for something productive.