
Hi LindaA - your boys sounds like he had a really bad case when he was young - if I were you I would tread careful - try the groomer and cornflour or cornflour & baby powder dry baths in between times and see if that works or helps - I try not to muck around with my boys diet and cleaning regime too much since he had allergies when he was young that stopped aswell but I really don't want it back so I don't change much if I can help it seeing as whatever he's having seems to be working. I'm pretty sure the same applies to you from the sound of it too :)
So if his skin is okay and you're not concerned that its in any way changing, inflamed, itching, raiserd, reddened, or the skin itself is discoloured or smelling then I would try to keep things exactly as you have been since he was a pup since that's what's working - afterall grease is easier to live with than the skin flares.
Oil of primrose may hydrate the skin but I don't think any grease on the hair will be helped unless the skin itself is the cause - you can try some if it has worked in the past and try a brush through with cornflour if he's in the mood to let you in between grooms. Try to figure out if you have changed anything since about 2 months before his hair got greasy - because if its a reaction to something he's eating it can take that long to create a reaction - or maybe a change in washing powder, air freshener, flea treatments (advocate makes my dogs back greasy the whole month - stronghold doesn't), worming tablets. Because any change could be the cause and often its the unexpected.
If you do try anything make sure you just try one thing at a time - so if you add a supplement don't change his food or bathing at the same time - otherwise if anything actually makes his skin flare you wont know what thing did it - and if something works you wont know either. Try your groomer - ask her if its possible for her not to use clippers on the back but to groom with scissors or a stripping knife on the problem area justo try to keep the top coat longer than the undercoat and keep the undercoat light on that specific area - sometimes with clippers all the hair is the same length and even if it doesn't feel thick it is harder for dead hair or skin to shed and let the air in - if she cant do that then make sure you do your best to slicker it through a lot as it will disperse the grease and help with the undercoat shedding easily. (hopefully your boy will let you do that?).
If I think of anything else I'll be back in the thread to say!! And if you do have to go to the vet - even if they say some rubbish about bathing him - at least they have put your mind at rest that its not medical. I found that vets take one look at a westie and the first thing they ask about is the skin and do I know that westies get allergies - drives me nuts - I've learnt to ignore anything they say unless its actually a diagnosis or treatment. Last time I went I walked in - told them that my dog had a skin flare up in his ear that's caused an infection and that I need Cleanaural wash and Canaural drops - vet told me that it might not be that or need that treatment - poked my dog around - spent 20 mins telling me about skin and the ear and how to administer drops - and then gave me the exact stuff I said I needed and confirmed he had what I said he had. Guess they have to find a way to justify the appointment costs :) That said - I always go when I worry since its worth the cash just to put my mind at rest.
Hope you find the cause and something that helps with the coat - from a fellow mum of westie allergy-sufferer - with love x