I've googled it and it seems it could be teething (he is chewing everything at the moment), compacted anal glands (boak), his food (he is having lamb flavour Wainwright's), an allergy, a yeast infection, anything really. Teething seems most likely ... I've even wiped his bum with a tissue and sniffed it (not fishy) so not sure about the anal glands ... things I never thought I'd do pre-dog!
Anyone had this before??