Not sure that there would need to be a commercial agreement between mother and son.
There was a comms decision on something broadly similar and will post again if I can remember it but, essentially, you need to look at the overall agreement. Despite the middleman there is, on the face of it, still a commercial agreement in substance.
For anyone who has a better memory then me (ie everyone), the case I am thinking of: A property was let to a charity who, in turn let it to a Yemeni (I think) couple with the LL’s knowledge. The arrangement served a dual purpose (a) The LL’s rental income was protected and (b) The couple were housed in close proximity to a number of other recently arrived Yemeni nationals.
Whilst there are some differences – LL’s knowledge and familay angle I do think the basic principles are the same.