When I am trying to grow a mane and tail and add general condition to a horses coat, I feed seaweed meal. I got dry kelp meal on Ebay
for half the price of Source and it is the same stuff. I feed about a tablespoon once a day and I have had great luck with it every time I have needed it. It is fantastic for hooves, too.
The oil is also a great idea because it sounds like he has dry skin. The TB I just got has that same dander in his mane. I have a bottle of hair oil that someone gave me, but I don't like the smell of. LOL I have been massaging that into the base of his mane when I groom him and that seems to be making a difference. I did wash his mane out really well one good time, I used a mild moisturizing body wash and mixed baking soda in since that will help take care of any possible infections or fungus that might be contributing to it. Just make sure you rinse it out really well.