Well I feel kind of stupid for asking this, I've owned dogs for years but never a Berner or even one with a heavy under coat. The last 3 weeks Brodie has been losing tons of hair. It looks noticibly thinner, on his butt and top of his tail it's a super wirey texture. Is he blowing his coat? Its been super hot here in the Midwest.
Normally when is turns warmer dogs will blow their coats if they are a double coated dog. We live where it is warm been over 100 degrees for two months. Ours has been loosing fur but you really couldn't tell it. No, skin shows through. If yours are loosing hair in patches where the skin is showing I would see your vet. There is some parasite and even allergies that could cause this.
As dogs age they can develop allergies. The first thing you need to look at is his food. If you switch to a protein based grain free diet and the issues don't stop he may have a fungal condition that will require a vet to diagnose and treat.