The things people will do...
My mother never passed a UK driving test (born '43).
She told her father she had passed, he bought her a van, she drove that around until she moved to (what was then) Rhodesia and got a licence there (but I doubt had to do anything like a british driving test).
On returning to the UK some years later she just.... carried on driving around on that south african licence.
She married my father and I would assume, they had an insurance policy in his name that covered her rather than in her name...
Eventually in the mid 80s she was stopped for not displaying a tax disc, and I was there so I remember this clearly... they looked at her licence and explained it wasnt legal, and then off we went home and my Dad sorted it out..
How, I will never know, but effectively her illegal SA licence was swapped for a british licence and she carried on driving, if she weren't pushing up the daises now she'd likely still be driving without ever having legally passed a UK test.
I'd stop the kids going in the car with him driving, and then I'd find out if he has achieved a licence without passing a test.. or is in fact driving without a licence at all.
If the latter, try and get him to pass a test I guess. If the former, not a clue what you'd do about that!