Hi Guys
I wonder if there is someone out there who could give me some advice. I am currently fostering a 5 yr old male gsd, he is absolutely adorable but he is in such a state. When he came to me he looked like he had been taken care of but on closer inspection it looks like his previous owners have taken good care of the front half of his body but the back end is a mess

I'm not making myself very clear am i

, he is long haired and his front legs and most of his back are nicely brushed and his tummy is quite easy to brush but when it comes to his back legs and tail he is completely matted to the point that i can't even get a brush in to start teasing out the knots, i don't want to stress him so i am trying to do a little bit at a time but getting nowhere. I wonder is there anything else i can do to help him, like a better shampoo or something to help de-tangle him and make brushing easier?
The brush that i use on my shepherds is not doing any good on him and i must admit that i rarely bath my shepherds as they get loads of brushing so i don't really know what to do for the best .
Any advice would be great