
First you need to collect the adverts so you can prove there are more than 5 litters in any 12 months.
Then you need to contact the relevant Local Authority and check if they have a breeders Licence and for how many bitches.
Then if it is obvious that regulations are being breached (for example pups have to be identified etc), then ask the LA to act on evidence provided.
The breeding and Sale of Dogs (Welfare Act) 1999 refers:
http://www.legislation.gov.uk/ukpga/1999/11/contentsAlso the Trading Standards department will be interested if the pups were sold as purebred, and are obviously not. Might be worth having a breed expert look at them and confirm they are not, rather than just being poor examples.
Of course he may be trading lawfully ;(