In reply to aln:
Sorry to hear about your son, I hope he's feeling better.
This period is one of the peak times for many respiratory viruses to be circulating in the UK. It could have been any number of viruses (RSV, influenza A, influenza B though that tends to be later for some reason, para-influenza, human metapneumovirus, picornavirus) that increase in abundance during the winter. Unless a specific lab test (not cheap nor routine) was done you wouldn't really be able to distinguish between them, not even by symptoms really.