Unfortunately your son would not be covered by unfair dismissal laws until he is employed for at least 6 months in a firm with over 15 employees.
The conduct you describe, however, very likely amounts to bullying or harassment and as such would be actionable and you can read about this here:
Whilst you could now simply go to a lawyer to see about bringing legal action against the employer, however, you may want to reflect as to whether taking legal action is worthwhile at this point, given it may disrupt his apprenticeship and may impact on your son's reputation in the industry. Obviously action would almost certainly be appropriate if he were sacked or disciplined undeservedly, but if he really is just being subjected to some idiot manager's joke, it may be that the better course is just to ride out the joke and in the meantime, make a point of collecting evidence of such conduct by retaining copies of any emails or communications detailing such behavior, and by keeping a work diary recording any such jokes, as well as recording anything that shows that his work has been good. Such could then be used to support a case at a later date should it ultimately prove necessary.
Note, I am not trying to dissuade your son from taking action now, merely inviting him to consider the potential impact of such action beyond simply securing compensation or a direction to the employer as to how to behave.
It may also be worth discussing the situation with the superviser to get some advice as to how to handle the situation with management and/or HR. A letter from the superviser in the form of a reference confirming your son's good performance may also prove useful, and may be more difficult to obtain after any dismissal action has occurred, but if obtained now may be very useful in establishing that he was not ultimately dismissed because of poor performance. A direct approach to management about this bullying behavior could also be considered, but until, unless your son can work out where these allegations are coming from, it may be unwise to deal with the matter this way, as if your son complains to the source of the complaint it will likely only worsen the problem.
If there is a union rep in the workplace it may be worthwhile discussing the problem with them, and if your son is not a member of the union, it may be prudent to join, as membership of a union will usually allow access to help and legal resources at concessional or free rates should they later be necessary.
I trust the above assists.
Good luck and PLEASE RATE MY ANSWER.