If you are looking for a spray that helps itching, I'd suggest espana SILK's protein antiseptic topical spray. It's fantastic.
He may just have dry skin from the weather changing. The blanket probaby holds in the moisture. Just like if we wear mittens, we are less likely to get cracky, dry skin in the winter.
I'd try either the silk product like this: SILK
Or I'd look into some sort of horse skin moisturizer or a neck piece for your blanket.
A lot of people will suggest MTG for itchy skin but I really dislike that stuff because it's not worth smelling like bacon grease for DAYS especially because there are products that work much better out there. I've also been told "healthy hair care moisturizer for horses" works really well :]