So it would seem on the road in question that he probably needs to be 1.3 metres from the kerb for his safe passage.
The police recommend 1.5 metres as the minimum safe passing distance from a cyclist, so a car must be 2.8 metres from the kerb at this point to complete a safe overtake.
You have an unusually thin son, on an unusually thin bike. In most case I'd suggest adding at least another 0.25m for the right half of the bike.
This is an attempt to use humour to remind people that despite what some drivers (and some cyclists) think bicycles have non-zero width.