My Oreo is going through the exact same thing.
She has 'hotspots' (red, sore, damp spots) between her toes. This is only on her back feet. She doesn't touch the front. She does tend to favoure one back foot but does go for two.
It is the time of year for dog allergies and if he's only just started this now I suspect it maybe something on his walk that may have upset his skin. Hopefully it isn't a lifetime allergy.
Allergies can come at any time in a dogs life.
Food allergies are a close second for causing allergies but are less common than airborn ones.
Personally, I would take the dog to rule out any issues with back end issues such as hip problems but as my vet told me only yesterday if it were this you would notice your dog walking funny and other things first.
Bathing the feet in warm water mixed with either a small amount of hibiscrub or salt water. This will wash away any nasties that have built up. Keep them soaked for as close to 15 mins as possible and rinse well with warm water.
Tap them dry and don't rub.
Have you changed anything like washing powder, air fresheners, shake-n-vac type products recently? If you have remove them first to see if the itching stops.
HAve you put anything on your grass?
Have you checked for fleas?
Once the above have been checked I would move onto food. What food is the dog eating at the moment?