It looks like a wasp nest to me too. I had one in a tree stump in a garden once and had to get the council out. It was huge and people were frightened to walk past it.
My sister had a huge wasps' nest in their shed. As a primary school teacher she wanted to take it it show the children. She sprayed it liberally with hairspray so it held together. The children could then gently pull it apart. Obviously the nest was abandoned and no live wasps!!
I would say hornets' nest. we get them here in Southern France quite often. They are huge and unlike a wasps nest you do not discover them until the leaves are off the trees as they usually build them high up in trees.
As said earlier, once the queen has gone at the end of summer they never come back, unlike bees. We had one in a baling machine.
You can buy “mock” wasp’s nests made of paper to hang up in the garden. Apparently wasps won’t go near another wasp nest so it keeps them away. Haven’t tried them though
We discovered one of those in progress in our garden shed some years back. That then explained why, when sitting quietly in the sun, I had heard a rasping noise and saw a wasp seemingly gnawing on the wooden garden fence. Must have been collecting material for the nest.
Very large wasps nest - I've had two of these in the eaves of different houses - the local council may come and remove this for you (at a charge). If not, it's a pest control company. The important thing is for your son not to try to destroy this himself.
I had a monstrous wasp nest in my hedge a few years ago. My local primary school were delighted to have it for their wildlife lessons. As your son appears to have a few, perhaps a local school might be interested.