If he's losing this much coat and it's not looking thinner then maybe he's just having a very late moult? the weather this year has been confusing for coats to say the least, varying between cold and very warm - maybe this is causing a heavier moult now?
If he's happier on stone floors then could it be that he is simply feeling the heat because of his heavy coat?
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Rubber Brush in the bath will help you get loads of coat out - and you can use it in between too.
There are numerous tools to help keep shedding under control, you may find one which works for you ;)
This Article mentions dermatitis in connection with AF and GSDs