One of the things that I like about trials is that I chose to do it myself. My parents never really pushed me to do sports or any activities, my mum also played the piano at that age and she still holds it against her parents for making her do it. Also he's not going to stand up to you about it most likely, so he isn't choosing to do it, although he may like it. And it seems quite dangerous to me, 4 year olds don't have the same amount of fear as us.
Still, good riding, but as good as I'd expect him to be considering he 'trains'.