Practicing this: Overall - Relaxation Protocol
first at home and then gradually moving outdoors might help if you believe this is an anxiety-based behavior. Get the dog really responsive somewhere he feels comfortable (like always in training), then move to just outside your door, then into the yard, etc.
Since his behavior is not triggered by any external stimuli you can see (as in, it happens at different distances, times, locations), once you start moving away from "safe" places you'll have to really watch him and implement the relaxation protocol at the first sign of anxiety. Have you noticed any changes in his body language before he begins jumping and biting, or anything he does leading up to it? That's the time to do respond, not when he's jumping. Basically, the relaxation protocol redirects him to a calmer state, but it's very hard to do if the dog is already jumping about like a maniac.
I also wouldn't rule out that it is inappropriate play, though even if it is the relaxation protocol may help (I've never used it for that, but I know others who have). That was actually my first thought when I read your post, especially since he grew up with his sibling until recently. Puppies who grow up together often play pretty roughly IME, just because no one has taught them manners!
I also don't think following you throughout the house is a sign of separation anxiety. It could be, but GSDs are also just like that. If you have other reason to believe he's anxious when you're gone then it could be, but if he seems fine when you do leave and just likes to be with you wherever you are when you're home, then I would lean more towards him just being a "velcro dog."
Do you play with him at home? If so, that's the time to start setting boundaries. No rough play, no mouthing, teach him specific games, that sort of thing, and make sure he gets plenty of opportunity to play. 17-month-olds still need a lot of it.
I walk a dog at a rescue who does this exact same thing, and it is play and excitement for her. I've found that simply taking her in the yard and giving her a good play session (with appropriate boundaries of course) before we go on our walk goes a long way towards fixing it.