I agree with FancyNancy...after living with multiple pus filled spots on my boxers chipchops (lips and chin), I wouldnt worry if it is only the one...it seems very common in younger boxers but Im not sure in c.spaniels...
See how it goes once all the pus is out and either use hibiscrub but if you dont have any warm salt water would be fine...if it gets worse or bothers Finn or more appear he may need some abs to clear it up.