Dropping the front quarters into a 'down' position while the bum remains in the air is known as a 'play bow' and is a very friendly gesture; he's trying to entice the other dog into play, and is also very clearly communicating that he's not a threat. So long as the other dog is up for it, just let them get on with it. They're only young once and deserve to have fun; by the time he gets to age 5 or so you'll look back fondly on the days of him playing with other dogs in that way.
Of course, not all dogs are up for playing for a whole host of reasons (being a bit middle aged, not in the mood, just doesn't hit it off with your dog) and your dog will need to learn some calming signals and what they mean en.turid-rugaas.no/calming-signals---the-art-of-survival.html
You would do well to do some research on dog body language, both so you can understand what your dog is trying to tell you, and so that you can read what the other dogs he interacts are feeling about things.
A non-retractable long line attached to a harness (not a collar, for safety reasons) is ideal for the stage when you're teaching recall but haven't got it nailed yet. It allows the dog a measure of freedom within certain parameters; when he's coming back to you reliably when called on the long line (give a treat each and every time!) you can start letting him off.
Tracker woman is nuts though. If a dog is stolen the first thing they'll do is to remove the tracker, and it doesn't protect the dog from being run over etc before its found. Has my dog ever failed to come back the second I called him? Of course he has - and anyone who says that has never happened to them is lying through their teeth. But in 5 years he's never fucked off to the extent I didn't know where he was actually that's a lie too, he got through the fence in a garden we were visiting, ran through multiple gardens and their holey fences at breakneck speed, round the block, and turned up on the front doorstep about two minutes later looking very pleased with himself. No one is perfect!