Bicycle Tutor

My son has a outrage trax mountain bike bought at xmas from Halfords and has had a problem with rear wheel rubbing on frame... has been back to halfords 3 times since on last time they said they put stronger wheel nuts on bike. Seems ok for about 2-3 days then old problem arises wheel not buckled any help would be appreciated.

There is no such thing as "stronger" wheel nuts. If the thread of the nut is stripping then the nut or spindle is faulty.
What kind of rear wheel fitting is it?
a) Solid spindle with nuts both ends.
b) Hollow axle with quick release lever.
c) Vertical dropout.
d) Horizontal dropout.
I have looked at a picture but can't see the detail of the rear spindle.
If the problem persists then the bike is still under warranty and either needs fixing or changing or your money back. A rear wheel suddenly slipping and catching will slow/stop the bike, (sometimes as you set off from a junction) and can cause you stop in the way of an approaching car.!!
This is a safety issue and must be corrected. (I know because it has happened to me, lucky though as the car managed to stop.)