I have 2 longcoat GSDs and last year the bitch had a terrible coat. Became very thin and dry. I tried loads of things, and in desperation put her on GSD 24 food by Royal Canin. Could be coincidence, but her coat is back to being beautifully thick and glossy now.
I use a rake for grooming out the undercoat. It is shaped like a t bar, and the rake has 2 rows of bluntish metal teeth.
Hi, When he has been with you a while, and a good diet, his top coat will grow through nicley, I would give him a tin of fish at least once a week, Supermarkets, tuna, pilchards, sardines, It will make his coat better.