help and advice please

[tigerlily11]

My son works for a shop selling computer parts and other electronics. They have shops all ove the country, some of which my son helped to put together. He is a salesman, one of their better ones (Sorry if I sound a bit proud of him but I am).
The problem is his manager has been putting him on shifts that mean he works 8 day straight before he gets two day off. This as you can imagine is getting on his nreves as the manager doesn't seem to do this to the other workers in the shop.
What I would like to know is, is the manager allowed to do this and if not what can my son do about it.
He doesn't really want to leave because other than the moron of a manager he doesn't mind it there. It is the first job he ever had and hes been there for about 5 years.
Thaks for your time.