If this is a "bicycle shaped object" from Walmart and such, with a thread-on freewheel (1970's design not seen on professional quality bikes, even at the entry level), the bike probably has broken rear axle. (Has the boy been jumping off things?) If the axle is broken, you can get a whole new wheel for that type of bike for some $30 US or maybe even less. This type of design uses a long, thin steel axle to connect the wheel to the frame. The rear sprocket cluster ("freewheel") screws onto the wheel hub and allows the axle to pass through it, but provides no support to the axle. Thus on the drive side, there is a lot of leverage acting on the axle. These axles tend to bend and break.
Or the axle cones could simply be too loose. On those bikes, you can tighten/loosen the bearing cones with a wrench. If they are too tight, the wheel doesn't spin freely; too loose and the wheel will exhibit side-to-side free play on the axle. There is a nice, bitter spot somewhere in between whereby the wheel exhibits no free play and still spins freely. You want to tighten it just a touch from there to pre-load the bearings (so the balls don't separate from the cone surfaces when the bike is under load). The people at the LBS would surely have diagnosed this, though.