It's really hard to know what is right. I don't think anyone makes ramp bikes that are light enough for a six year old to enjoy. A Cnoc 16 will do, but even they are heavy as hell. If he was riding dirt, a jr race bmx would at least be light enough for him to manhandle in a way that is fun. Ramp bikes are so heavy, they make the kid a passenger.
My 10 year old has a Islabike Beinn (sub 8kg with cheap carbon bits), a WTP BMX, and a Grit stunt scooter. In terms of making use of the skatepark, it's the scooter that's taught him the most.
And the scooter skills have transferred to the bikes.