That's a GREAT video, Jason.
Notice how Jason swings his hand back then forward to bring Ike back into position (at 34 and 52 seconds). This is an important move, so watch how he's doing it. What you'll be able to do, if your dog gets out of position, is use this arm swing hand movement to get your dog back into position.
Somewhere else on the forum, someone asked about how to train your dog to heel on the right side. This is how you'll do it, but with your right hand (so don't forget this trick)
I train much the same way. The only thing I add to that is I train (separately) a touch command, where they learn to touch their noses to my hand. Whenever I put my hand out and tell them Touch, they look for my hand and bring their nose in. It's great for helping them on a heel -- if they get ahead of me, then they will back up to where I put my hand, which is at my hip. And it's also useful for focusing them when there are distractions. I can put my hand in front of them, and then draw my hand (and therefore their heads) away from the distraction and toward me, where I then work on a Focus. https://www.germanshepherds.com/forum...ge=1#Post35064
If I just ask a young dog for a Focus and he's keen on something else, it's almost always NOT enough to get him to turn his head, but if I give him something else to focus on (my hand) and draw him away, then I've got him.
To teach a Touch, you can either simply put your hand out flat, kind of bop your dog on the nose and give him a treat, which kind of works ok, but takes longer to train. But a lot of trainers teach it this way. Or, you can train it (the way I prefer) by putting a small bit of jerky type treat in between your index and middle finger, but down by your palm, and showing your dog your open palm. When he brings his nose toward your palm, open your hand just enough to give him the treat. Repeat often, with both hands. Then, when he seems to get it, fade out the lure and let him Touch, then you reward with the opposite hand.
The first training approach is more passive. Dog does nothing but gets rewarded. The second, the dog has to seek out your hand to get the reward. That's why I like it better.
Once you have a dog that understands a touch command, you can use your open hand to move him around. If he's too far forward or too far back, just put out your hand and move it where you want him to come. You won't have to use your leash to guide him or have to manhandle him again.
One more thing about hind-end awareness -- young GSDs do take quite a while to get it. Mostly, they move their front legs and the rear legs follow so anything we can do *on the ground* to help them realize they actually have rear legs that can move independently is a big help. If you have a ladder (I especially like metal extension ladders) lay it on the lawn, leash up your pup and slowly lure him through. A helper is very useful, because he will almost certainly try to hop out the other side. Make him walk through the openings.
If you live by a school that has tires for the football team, use those. You can buy or make a wobble board (the Agility section has instructions on how to make one). You can get a 2x12 (12 ft long) plank at Home Depot and toss that on the ground and let him walk on that... Anything that is very low to the ground and forces him to move his back end *consciously.* All of these are great for improving the neural connections in his brain; they'll help with obedience skills like heeling, and if you ever want to work him in in agility or other sports, he'll be far ahead of the curve.
Finally, in response to your last question, no it's never too late for socialization. I think that dogs should continue to get out and go to new places, meet new people and experience new things their whole lives. It keeps their brains sharp, builds confidence and makes their bonds with their owners stronger. The experiences should be positive ones in new interesting places -- not the same old places, which are familiar and therefore comfortable. So get out there and have fun!