Is a "run-bike" equivalent to a bmx bike with no drivetrain?
Are you thinking to get a small BMX and take the drive train off instead of getting a run bike, so you can put the drive train back on when he's ready? Otherwise I don't quite understand the purpose of the question, sorry.

If that is you're idea I'd suggest that's probably not a good idea. It's going to be difficult to find a BMX small enough for a two year old to easily put their feet down and run, and you're paying for extra components (chain, cranks, pedals, brakes) that you don't need. Unlike an adult bike those components aren't likely to be easily removeable, and it's likely to be a lot heavier and more clumsy for your son to use.

A run bike is going to be focused on what is most important at that age - teaching balance - and everyone I know who has had one has enthused about how easily their child was able to transition to a bike with a drive train after having a run bike. Also, because they tend to be quite well made and not abused, there's a healthy market in them on ebay, so you should be able to sell it relatively easily when it's time for the first proper bike.