It's really a bit too late to do anything about this now I should think. If he was told he would be receiving full IKA then he should have around 25 to 27 stamps per month for the months he worked (there is a maximum monthly number that can be paid, so even if you work every day for a month you are not allowed 31 stamps per month).
There is cut off date for paying the stamps is monthly,usually at the end of the following month. So if your son finished work at the end of October, the cut off date would be the end of November to pay that month's stamps.
I think it is possible (but I'm not 100% certain) to supplement a small number of stamps with IKA for health cover and pension purposes (but not for unemployment benefit) so I would advise you to get down to the local IKA office as quickly as possible. The cut off date is likely to be the 31st December for this.
In general the only other way to supplement stamps is via a 'friendly' accountant, who may add you as an employee of one of his other customers if they have a 'space' for an employee (so not strictly legit). You would then pay the full IKA contributions yourself directly to the accountant, month by month during the season.
Your son's employer has acted illegally and could be reported to the IKA authorities for non payment and they will be in serious trouble. At this stage though it is probably too late for your son to benefit from this action so you should consider whether or not it is worth doing this.
When working in a seasonal job or for a new employer you can check that your IKA contributions are being paid by going to the IKA office and asking for a print out of your contributions to date (2 months in arrears) and I would advise everyone to do this if they are not receiving monthly wage slips showing IKA deductions.