The bike in the Instagram video is a Diamondback Mini Viper. He got a balance bike (the Strider) for his second birthday and like your son, he also had a tough time standing over it. But, he grew and was able to get up on it pretty quickly. First, we practiced locomotion and later, coasting with his feet up on the stays behind the seat. Once he was flying around on that bike, we switched to the pedal bike. We had the same problem--he wasn't quite tall enough--but he grew again and could straddle it, sort of. The actual transition from balance bike to pedal bike went really quickly. Main thing they have to learn is the coaster brake.

Good luck.