10 Year Old Australian Shepherd Getting Fatigued

My boy Foster recently had his 10th birthday last Oct. Prior to that time he seemed to tire very easily. Being my first Aussie, I'm looking for experience to answer question about aging, I guess. We exercise most every day, in the beginning it was all out all the time. Now, he tires after two chuker tosses and lays down panting. This is the way we exercise. Even when we are doing nothing but usual day time stuff, he wants to lay down much more than in the past. Speaking of the past, am I living there and my boy has just slowed down or what? I just had a full exam, blood, xray, etal, with no negative report. I am currently planning on having him fitted with a halter that will monitor his heart rate to determine if there are any issues regarding his heart.

Your thoughts please, thanks Nick.