It takes a lot to get a gross misconduct allegation to stick
Not necessarily - While gross misconduct is usually a single, extremely serious breach of the company code, it can also represent an accumulation of much less serious indiscretions. This seems to be the case here, although we can't say for certain.
We have a lot of employment protection in the uk.
True, however OP's son has few protections in place due to their length of service
In theory, I could sack you for Gross Misconduct for looking at me funny. With over two years service you would of course go to a tribunal, which would almost certainly find it nowhere near the GM threshold, and rule in your favor for unfair dismissal.
With under two years' service, OP's son has no right to tribunal, unless his dismissal was based on a protected characteristic or an 'automatically unfair' reason, such as asserting a statutory right.
Here's where this could get interesting... Wrongful Dismissal (for example when your employer breaches your contract by not paying out your notice period) does not require 2 years service to claim at tribunal.
GM usually means that the employer doesn't have to pay notice, but a full and fair investigation must be carried out to establish this. If OP's son believes that the investigation failed to establish that his conduct amounted to Gross Misconduct, or that the investigation wasn't fair, he may have grounds for a tribunal.
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