It depends upon what you mean by 'best'. I was regional secretary for the Professional Association of Teachers for several years, because I believe in their policy of always putting children first and refusing to go on strike. PAT has now become Voice:
http://www.voicetheunion.org.uk/
(Voice's strength is largely within primary education, but there's a growing membership within the secondary and tertiary fields. The union is sometimes referred to as 'right wing' by the media but it's constitutionally an apolitical organisation).
If your son's political views are
strongly to the left, the NUT might be the best union for him:
http://www.teachers.org.uk/
(The NUT is well-represented within both primary and secondary schools).
The NUT's big rival within the secondary sector is NASUWT:
http://www.nasuwt.org.uk/index.htm
It tends to be favoured by the
Guardian-reading brigade. (The NUT is closer to the
Socialist Worker)
The Association of Teachers and Lecturers has its roots in the old grammar schools. It was formed by the amalgamation of the Association of Assistant Masters in Secondary Schools and the Association of Assistant Mistresses (plus a later name change). Members are almost entirely within the secondary or tertiary sectors, particularly in the remaining grammar schools or in tertiary colleges. It also has plenty of members in the independent sector. Like Voice, it doesn't side with any political party (other than on specific issues relevant to education).
http://www.atl.org.uk/
Chris