im not sure i get what you are asking, especially about the bmx style cog thing....


this might be something you already know, but here we go:
most fixed gear/singlespeed back hubs has one side you can run a freewheel cog, and the other side you can run a fixed gear cog held on with a lockring. these are called "flip flop" hubs because you can flip the wheel around to go from freewheel (coasting) to fixed and vice versa.

if your son is new to fixed gear, definitely make sure it has AT LEAST a front brake, and i would advise to have him get used to the bike first with the freewheel side then flip it to fixed once he is accustomed. and make sure if running fixed the peddals have foot retention.