Agreed with MaggieRoseLee.
I took Rocket last year on an 8 mile hike in October--he was born in June, so he would've been 4 months. We were prepared to carry him, but he was fine. We stopped a lot and he would rest when we did. We hiked several times a week all fall, winter and spring, doing up to 8-9 miles.
This summer we've backpacked him three times. One a 5 mile each way, one a 10 mile each way with lots of talus slopes, and the last one was about 4 miles each way, but he actually packed his own stuff this time and it had a killer last mile of very technical and rocky, steep trail. (There is a thread in the pictures forum with pics of it
The pack we have is a bit too big, but it worked for that one. The first time I put it on him, it was empty and we immediately went for a hike so he would associate it with his favorite thing to do. He frankly only wore it twice, empty, before we packed with it. I only loaded it with about 4 lbs of food, so he maybe carried 6lbs. He was fine. I wouldn't overthink it too much, they're pretty stout dogs, as long as you don't try to make him pack 20lbs his first time out or anything.
Is PT physical therapy? Is this for his pasterns?