My 3 year old rottie is looking thin round the rear end he eats well and lately seems to pant a lot

Thin around the rear probably means, (with you looking down at him) that he is probably too big around the middle. If he is panting a lot then he is carrying too much weight; he eats well could be eating too much. Time to put that lovely rottie on a diet, especially now at 3 when he can work it off physically.

Tick

No he has definitely got thinner around the rear the last week or so

Thinner? As in muscle atrophy?

If your dog is losing muscle tone, then you need to take your dog to the vet. That is not normal.