Yes, he can sue for the money, though he may wish to first contact the state department of labor to see if they can help him get the money to which he is entitled: their help, if they provide it, is free and also expert (they have people who go after wage and hour violations for a living).The law requires that hourly employees be paid overtime for all time worked past 40 hours in a week: when an employee is not paid the overtime to which he is entitled, he may take legal action.