Hi there and thank you for this question.
I am sorry to hear about this behaviour and appreciate your concern.
As it happens when people are over, it is likely he is reacting to increased stress or anxiety levels. He is trying to deal with his extra adrenaline by using up his energy.
He may well be getting some comfort out of what he is doing; a form of self soothing.
I'd ensure he has a safe 'hiding space' he knows he can go to when people are over. This should be a place where people (including granchildren!) are not allowed to approach him.
This may be e.g. a cat bed up high on a cat tree or shelf.
I'd also consider having him in another room when people arrive and only allowing him in if he directly chooses to come in and join the people.
I'd have a Feliway plug in near his safe space and would also start him on some natural calming supplements such as Zylkene.
In general, we want to burn off his energy in productive and healthy ways. This means daily play sessions, with things like laser pointers, food puzzles, feather toys etc.
This way, he is getting the mental and physical stimulation he needs and will have less energy to chase his tail.
We must never tell him off for tail chasing as this can increase anxiety and make things worse.