I have found Omega Horseshine to be a really good skin and hair supplement. Its available in SmartPaks or at Tractor Supply. I give my girl 1/3 cup daily and her mane and tail and coat look good. Recently we have had itchy dandruff problems as well and my vet says it’s from no-see-ums biting her at night. We live in FL. She had a gorgeous thick mane and tail but now its all crazy patchy. I’ve started doing a 1 to 1 ratio of 10% permethrin livestock spray concentrate and water ( so 16oz permethrin to 16 oz water in a spray bottle). It has really helped. My vet says its the only thing he has found works to keep the no-see-ums off.
As far as hair being rubbed out in winter, check his blanket. Pretty much all blankets will rub the mane when on. While I am not a huge fan of ShowSheen, it does really help prevent blanket rubs. Its about the only time I use it.