I am concerned about the level of activity our six month 55 lb. pup seems to be able to handle. He walks in the morning but throughout the day he just about refuses to go outside except to go to the bathroom. We try to walk him but he will walk long enough to pee/poop and then "plop". Won't get up until I head towards home. He will eat dinner and then get his energy up. We play/train inside for a while and then we will walk. Lately, he will walk till he goes the bathroom, takes a couple zoomies in the sand traps (we live on a golf course) and then wants to go back home. The walk lasts only about 15 minutes lately. Comes in the house and he lays down. Very rarely can I get him to go on a longer walk. It has been warm but I feel he is just way to calm for a puppy. Am I being overly concerned that there could be something wrong with him? Heart issue? He was very well bred buy a reputable breeder with all his clearances and our hope is for him to become a therapy dog.