Hello, and welcome! To be honest with you, at your dog's age, mine was going on and still goes on just about daily off lead hikes through rolling hills that she loves bounding up to the top and full speed back down to the bottom. There were no boulders really so I would try and monitor that if you can (you don't want him making any HUGE jumps down, but as long as it isn't repetitive and excessive I really think he is fine). Hikes like these I didn't worry too much about because she is setting her own pace and the changing terrain isn't like taking a dog on a pavement jog where it is being forced to run at the same pace on hard ground for an extended period. If you are like us, and have the luxury of nearby off lead hiking....I say utilize it! Bella has great muscle tone (more than some of her other littermates from what her breeder says) and I believe her hikes really had the most to do with that.
Training at home is also a great way to expend energy. Sometimes mental exercise can wear these guys out just as much (at younger ages especially). At that age we had daily training sessions with Bella, that I often incorporated breaks for play. So I would train (she would earn play) and then we would play a bit, switch back into training. I still do this. Yes, they LOVE to learn (well from my experience). If I am teaching Bella a new trick you can just see the focus as she tries to figure out what it is exactly we are learning. And those "light bulb/Ahah" moments are priceless. Bella knows sit, down, Nose (nose to ground in a down position), roll over, army crawl, sit pretty, stand tall, wait, stay, here, "circles", what my husband taught and dubbed "Ole" (she goes through the middle of your legs), heel, jump, hug, up up hug, Wait nose (treat on nose), go settle.........There's really no end to tricks and useful things you can teach a GSP I still want to teach her to cover her face, but haven't gotten around to it. Hide and seek is also a fun game we play inside that is especially great for your pup if you are working tracking. Bella waits in one room while I hide a toy anywhere in the house. I return to the room and tell her "go find" and she goes to work. I throw a ridiculous party for her when she finds it lol. She LOVES it.
Enjoy this age, because before you know it they are all grown up and no longer gangly adolescent puppies lol.