My gsd is almost 7 years old and virtually from day one he sleeps in the shower stall. He has a nice crate, he has the run of the whole house, he has one of those gel mats next to our bed but where does he sleep? In the shower on the hard tile , wet or dry, winter and summer. Well, winter here in So. Fla. is a relative term and I do think he likes it because the tile is always much cooler than any other surface other than his gel mat. I think the squish factor of the gel mat is a turnoff to him.
I'm sure your pup will be fine. Be sure to use some cream on his elbows-this would be preventative advice for any gsd laying on hard surfaces regularly. I use Penaten.