Thanks for all your suggestions, you lovely people I will go to gp. Don't think it's infected, but don't want it to become so, either! Don't think it's teething - he has two teeth that appeared quite suddenly and without any (other) fuss or drooling, but really, who knows?
My ds's weight has always been on the low side but has finally crept up to 2nd percentile line now that he's being supplemented with formula.
Thought he was developmentally advanced (sitting since 5 mos, and now walking forwards on hands and rocking bum up in the air) because he was small, but mamagoose - wow!
Have just finished second week back at work. So hard. DH has taken 6-month leave of absence to be with ds, which is marvellous, but makes me so jealous when I leave in the morning and they're playing on the bed in their pyjamas ! Luckily have Fridays off to reclaim ds!