Depending on how much cash you have the atomik will be okay for his considering he's only a little fella and will not be to harsh on the machine however ducar's are not the pick of the chinese stuff many riders on here perfer lifan ,gpx or jailing motors to be the pick of the bunch though the ducar motors can be good however tend to vary , my mate has a black ducar 124cc motor and he's absoulutley given that thing a run for its money with next to no oil in it ,doubling around people around steep country and riding 7 hours on it almost straight and the thing still fires up within the first few kicks and is extremely reliable.But its the inconsistancy that lets these motors down.
If you could spare parting with another $400 you will be able to pick up a second hand jr50 ,pw50 ,kdx50 or if your lucky a xr/crf50.I would try to get something that will last perferably japanese so that your son can hand it on to his sons 30 or so years down the track.