All of the travellers' sites in Norfolk (just like everywhere else in the country) will have a waiting list that's probably several years long. Trying to get a pitch on a travellers' site is like trying to get a council house; there are all sorts of hoops to go through, with families with young children (for example) taking priority on the lists.
Further, to the best of my knowledge, Norfolk hasn't got any 'transit' sites for travellers to use for relatively short periods of time. (Local authorities are meant to provide them but hardly any do. The only sites that they have are for semi-permanent occupation so that, for example, travellers' children can go to the same school for several years).
As Maydup says, very few campsites are open this time of the year, even when there aren't any problems with Covid rules. Currently though, all campsites have been required to close unless, perhaps, they're providing accommodation to essential workers who need to move across the country.
Campsites which are notionally open in Norfolk at this time of year are listed here:
https://www.ukcampsite.co.uk/articles/view.asp?id=385&rg=East%20Anglia
However those are just listings providing the usual opening dates of the sites. They don't take account of the current lockdown situation. It might still be worth phoning around those which are listed as being open anyway, to enquire if they can accept a booking from someone who is effectively between homes. (Hotels, etc, are permitted to accept such bookings, so you
might find a campsite offering similar flexibility).
Otherwise, as Maydup says, it will have to be an informal arrangement, with your son parking his van on someone's driveway (or similar).