I love those pictures of the bubbles and the pvc rope toy! What terrific ideas! Those dogs are having a blast and getting some energy out! Fantastic!
I go on hikes with my dogs, but I live in an area of wilderness and there's lots of places to go, plus I can drive, so I can go to different trails. But, you can still run around your yard and try getting him interested in a game. Maybe you can modify the fetch game so that he'll stay interested better....hmmmm....let's see, what about running away with the ball and acting like a fool? LOL. Make him think you're having so much fun and he's missing out if he doesn't run after you. How about riding your bike around the yard and tie something to the back that entices him....like a racoon tail? LOL. (I saw that on Where the Red Fern Grows when the boy was teaching his coonhounds to follow a scent)
If you search high and low for a motivator to make your dog more "into" going on a leash, he should come around. Just make it fun, get treats, add a throw of a ball to assoicate good times with being on the leash. Try walking a short distance, part way around your block and make a game out of it. Try not to get discouraged. He'll grow up and get use to stuff. I've said this before, but I'll say it again. I had a horse trainer help me with a semi wild Arabian mare. She thought she was being murdered every time you did anything with her. This trainer said, "whatever she doesn't like, do it more." And by golly, she was right. (just don't frighten the animal, but practice things more that he doesn't particularily go for.)
About socializing...I had Lyric as well as my GSDs (in the past) out every single day, meeting someone new, seeing new enviornments, all kinds of objects, different ground surfaces, noisy traffic. With Lyric, I had to drive an hour to go to a decent sized city to expose him to all kinds of things. I wanted lots of people to pat him. I drove to the city about 3 or 4 times a week and the rest of the time, just around here, down to the little corner store or to the cafe to visit. He saw lots of dogs and went to puppy obedience and puppy agility after that. He was very socialized to dogs and is now dog aggressive. So, some dogs will not like other dogs. That's just the way they are sometimes, socialized or not. BUT, with people and unfamiliar things in the enviornment, I am of the opinion that saturation of socializing is vital, not that you overwhelm or frighten the dog in any way, but a steady diet of getting out and about and meeting lots of strangers in a variety of situations and enviornments, as well as at home is extremely important, especially with a protective type dog like a Doberman or a GSD...or many others.