When will my Rottie/mastiff mix stop thinking he is a puppy?

Dogs don't have the cognition skills to 'think' they are a puppy. You simply have not trained an energetic young dog properly. While 'walking' is supposedly some magic fix for dogs, it does not train them to behave properly - just to be tired and while it makes some tv personality look good, it isn't training and your dog lacks training. Get back to the obedience cles and train him well in all situations. I primarily train my dogs in obedience in my home so that they behave well there instead of just taking them out to train them like a lot of people do. I also take them out but since most of their time is spent in their home with me, that it where I want them to behave most of the time. Dogs don't generalize well so go to the obedience cl, learn how to properly train him and don't wait for him to get too old and infirmed to misbehave.