Hi Wee Quinnie,
You don't say whether he is planning to come back to stay permanently or whether he just wants to stay a night or two and if so, why.
As far as the rules are concerned:
If permanently I think it is fine. He's essentially returning home rather than staying away. Moving house is also allowed [www.legislation.gov.uk
] (6L) and students are allowed to return to the family home for the duration of lockdown.
If just for a night or two it's different and really depends why he wants / needs to do this. I think there would need to be a very good reason.
How you feel about it is a different matter & I guess that depends on your own risk factors.
I think it would be fair enough to ask him to take extra precautions for a period of time before returning (median incubation period is 5 days but it can be up to 14).
Once he is back you might want to keep 2 metres apart for another period &/or ask him to do some extra cleaning. More info here [www.germdefence.org
But that is very much down to your personal feeling.