Many people recommend contacting the Citizens' Advice Bureau for help with debt problems. Your ex can find his local branch here:
http://www.citizensadvice.org.uk/cabdir.ihtml
However many CAB branches currently have a long waiting list to see a specialist debt adviser. A quicker alternative (which I've used and can recommend) is to contact CCCS. It's a registered charity which is dedicated to providing support to people with money problems. They've got an online help service here:
https://debtremedy.cccs.co.uk/
Alternatively they can be contacted (0800-2000, Mon-Fri only) on their free helpline:
0800 138 1111
Chris
PS: The current minimum wage (for a worker aged 22 or over) is £5.80 per hour. If your ex is only receiving £5.50 per hour, he should complain.