I am a lawyer in Scotland. Can you give me a little more detail please including how long your son had worked there?
Normally your son would need to have to been employed for two years to make a claim for unfair dismissal. However, from what you say your son was dismissed because of his disability and that is discrimination. No length of service is needed for a claim. Your son should see a lawyer immediately about this. A claim has to be made as soon as possible.