Thank you both, I think it is magnified by the fact I've been, to some extent, doing nothing most of the year (unintentional gap year) so I've been available to see him every day and we sleep round each other's houses pretty much every night.
I've tried to suggest positives for him that he'll have moved into a house with his friends, he'll be busy socialising with them in the evenings and there'll always be someone about he can talk to face to face (as he also said he was worried about being lonely).