Sometimes my Grandchildren work for me. They are all on the books the days of casual payments are long gone ask any paper boy / girl
Do you mean in regards to to employers no longer offering work on a invoice basis cos theres over 500m "bogus" self employed in the uk
Restaurant workers hairdressers all been told what to do no ssp no holiday pay its shocking!
But in regards to the OPs son doing a day and a half worth of cheeky window cleaning on an invoice basis I'm sure he'll be fine. Who better to ask than HMRC give em a call it's not breaking the law at all they will confirm!.
An accountant advising you to do do PAYE remember is in his financial interest so careful if they say so other accountants will tell you the truth and say it will be totally fine on invoice.
There was a high-profile case kicking about Pimlico Plumbers bloke on self-employment even earned £200,000 a year. Went to court and he lost as he tried to take holiday pay and sick pay he tried to argue that he was employed he had a good case to be fair but no cigar.
It was very much in the open and HMRC did not show interest in pursuing the company in National Insurance contributions or likewise disallowing his invoice on their expenses
So considering that your son's doing a little bit of window cleaning on invoice I'm sure will be totally fine as long as its a genuine necessary expense of your business.
Every accountant will advise you just to keep very good records and I would take a step further and just make sure your son is registered for self employment and make sure he does his self assessment.