One of mine is a greasy stinky and I use wall dirtie beastie which smells amazing. One of my dogs used to have food related skin condition and sorted now she is on different food the vet gave me special shampoo but she rolled in fox poo and it wasn't strong enough so I use dirtie beatie and then her special shampoo to finish. I would definitely ask the vet if this is ok as it is not normal for a dog to smell this bad when washed regularly. May be worth trying another vet as sadly some are crap.
Can you take the dog's collar off, wash it and leave it off? That might be fairly smelly too. My mum leaves her dog's collar on all the time and it reeks. She comes to stay here, and off it comes. Smell has gone.
Is the shampoo Malaseb - have heard this is what they treat Westie skin with - could you perhaps take him to a local vet and see if there is anything more they can do? Your FIL might have got used to the smell and doesn't realise how bad it is... good luck, it must be pretty awful even though at least you can have windows open in this weather.
My father in law has just come to stay with us for 3 weeks with his 13 year old Westie. Poor dog has a skin condition and despite having special shampoo and being washed regularly ABSOLUTELY STINKS. To the point where it is making me feel sick and I'm not sure I can cope with 3 weeks of it. Any ideas on how to drown the pong??? I'm sorry not to be more sympathetic but I can't be in the same room as the dog at the moment...