I would say that getting as much experience as possible is vital and preferably in different key stages. It's also good to volunteer at a school which has a diverse intake so he would gain knowledge of SEN, EAL, etc. The best course to go on is probably one sponsored by the school once he has a job - bit of a vicious circle really! Have you heard of the Helping in Schools course? It's run by the WEA where I live. I did it about 8 years ago and it was excellent. You needed to be an unpaid volunteer to do it and I found it invaluable. Good luck.