Boys are 3 times more likely to be affected than girls. The tic may be present all day but disappears during sleep or when the child is very absorbed in something. Stress undoubtedly makes them worse and your son's tic may be related to his start at school. They may also be made worse if attention is drawn to the problem. Many children find that if they try very hard they can suppress the tic for short periods, but this can be quite stressful in itself, especially as the tic may be a way of discharging tension in the body.
Most tics disappear within a year or so.
All you can do to help is to find out what might be stressing your son and help him to deal with this. Try not to make too much fuss about the tic itself. In a few cases the tic persists for much longer than a year, sometimes into adult life. But it usually remains as a very minor problem. Very rarely the tic becomes a more serious problem that needs medical help.