He looks about the size that Navarre was at 6 months. Yes you are going to have a big boy! He should stop gaining now and start getting longer/taller. Do you have him on any supplements? My vet just told me to be careful of the fast rate of growth because of hips, etc. I'll have to find some pics of Navarre when he was younger and post them.
On the flip side, my new GSD pup is just now 5 months old and I don't even think she weighs 30lbs! I've only had her 3 weeks and she's such a quick learner! But she was in the shelter for about 5 weeks and I think she's probably had the runs for the last 2 months (which have finally cleared up, seriously like yesterday!!)! Plus she's female and I think Navarre was part mutant!!! I will just have to wait and see!
He is a GORGEOUS pup!