Just a consideration, but sometimes frustration is a very good learning tool. Children generally learn to pedal around the time they start using the toilet successfully (I say generally, not that this is a universal rule) because both have to do with muscle coordination. There's something to be said for waiting until our children are ready for the next step...
The reason I mention this is that it's been my experience as a nanny that young children on *fast* wheels at this age can be a safety issue. Trikes can be difficult to master, but they have some great advantages over scoot/balance bikes.Many quality brands often make their trikes bottom-heavy, which can help to limit accidents and tip-overs. Because of their weight, it's difficult for the kids to really get up a head of steam, unless they are going down a hill. My experience is that the lighter toys like scooters and balance bikes allow very young children to go very fast--without the advantage of brakes or the experience of stopping a fast-moving ride toy. I have had to chase down a lot of kids who got beyond hearing "STOP!" simply because their wheels go faster than my voice (and because, with a helmet on, their hearing is more limited.
All this to say, a seated low scooting toy, which will help with steering and balance, might seem like a step backward, but it may be more on par with a toddler/early preschooler's abilities. As someone who works with kids, I see more and more often that products are being marketed to children that aren't developmentally appropriate. It's my opinion that while scoot bikes are a nice transition from trike to bike, they should be thoughtfully introduced. I've seen a lot of sad accidents and near misses just in my neighborhood because of youngsters on scoot bikes going too fast for mom and dad. We want to empower our kids to feel competent at something, but once again, frustration is a great motivator and mastering a challenging task is a great life experience. You can also practice balance with your son on park balance beams, too. Just remember, if you do get a scoot bike--it might be best to take it to a flat school playground or paved area for lot of practice away from the roads and hills!