
Atopic Dermatitis is basically a skin reaction to an allergy. If it is AD then your pup will keep getting it until you find out what he is allergic to and eliminate it from around or in him.
One of the most likely culprits is pollens / dust mites / fleas / harvest mites / storage mites. If you give him Piriton and see an improvement in the amount he scratches then it is likely to be a seasonal allergy like pollen.
If the culprit is something in his diet then the most likely culprit is going to be grains. There are some great grain-free foods out there. My little boy is allergic to chicken meal and other things that are commonly in a lot of foods and always seem to be in Science Plan or Royal Canin. I have him on Orijen Regional Red which works really well - although he has put some weight on with that food so I am adjusting the amounts. The other one my boy did well on was Burns Venison & Brown Rice.
If you cant figure out what is causing the allergy at all then the vet can take some blood and run the York Allergy Tests which will give you a list of any and all things your dog is allergic to. This is great if it only comes up with one or two things (likely to be the case with a lab more than with my breed Westies) because you can immediately eliminate it. If it comes back with a long list of things your dog is intolerant to then you will still have to do a bit of detective work as the list wont tell you which of the things he is intolerant to is the actual one causing the skin reaction - but it will still be helpful. In the event that you cant eliminate whatever it is from your dogs surroundings or diet then you can then undergo a course of immunotherapy - which is a serum made specifically for your dogs allergy that is injected monthly to build up a resistance to that allergy over time. My girl was allergic to some common grasses and after 9 months of immunotherapy she hasn't had a skin reaction since - which is about 7 years clear since she had the immunotherapy when she was a pup.
Lots of dogs suffer with allergies - westies are very bad with it - its a hard thing to watch your dog scratching and inflaming his skin - but bare with it and you will find out what helps your dog in particular in the end.
Some supplements can help too - like Evening Primrose Oil - and Dermol o3 is a new one which seems to work really well.