Number one for your son, join a union, and get some proper support in the situation he faces.
You don't say whether the company is large with many branches or just a single small firm. The size of a com-pany may have a bearing on how a situation like this can be resolved. For example, in a small firm if you can get a sympathetic hearing with the owner, you can get resolution quickly, but it isn't always that easy.
Large firms usually have written policies and whole HR departments to resolve grievances, but again that does not always mean that fairness prevails.
Anyone who comes in to support your son will want to know exactly what has happened, when, and whether anyone else witnessed it.
You and your son need to make a retrospective diary of what has happened so far, and keep it updated. Do not keep this document in the workplace or tell anyone else about it other than your union or legal rep.
This diary will form the starting point of any advice, support and actions towards resolution.
Bullying takes a myriad of forms and can range from obvious threats to cold-shouldering.
Try this link:
http://www.workplacebullying.co.uk/ - it was created by a vloke who was badly bullied at work, but turned his experiences to towards helping others. Sadly he passed away, no doubt as a result of the stress he was subjected to.
Here's another link:
http://www.bullying.co.uk/adults/index.aspx?_k k=work%20place%20bullying&_kt=9a1af211-47c1-41 41-964a-6009c2efc6ae&gclid=CITP_OXqoZgCFYoH3go dyWtRnA
Don't suffer alone; if your son has got to the point of feeling violent get him to the doc's