German Shepherd dog

We just noticed our 4 month old male running kind of funny.  Not sure if this is normal if there is something that needs to be looked at.  It's hard to explain but when he runs, his back legs look like they are hopping more so than strides.  He walks ok and doesnt appear to be any limping, just when he runs, looks like he is hopping.  He is a indoor dog and we do walk regularly and have him outside but we don't have him running too much and maybe he has always been like this but we never noticed?  He is a almost 50lbs at 4 months old and he checked out ok at his last vet checkup.  So is this normal or should we take him in?

Take him to your vet.