So I'm 4 pages late to this thread, but Willy this is what I have observed and done with my puppies and a few I've worked with/raised/fostered....
How old is he again? Has he teethed yet? Neither Nikon nor Pan were big tuggers until after teething. Nikon would chase and chew stuff, Pan actually had almost no prey drive whatsoever until about 6 months which is pretty ironic knowing him. Anna, the WGSL puppy I raised for a few weeks, actually had the most tug drive with the best targeting a grip of all the puppies and young dogs I've had in my house. Luna my foster was a lot like Jinx, she would go for toys and chew at them but not really latch on a tug. I think this is partly because the drives are still maturing and partly because some dogs just won't cut loose when they don't feel really secure and confident and/or might be somewhat distracted. I believe the latter is the case with Luna, since by the time she was adopted I could swing her around latched to a tug, but even now 6 months later at flyball she alternates between tugging really hard and being really chewy or only showing a mild interest.
I've found all my dogs and Luna too have done better when they have something specific to target, so instead of using long braided fleece tugs or bitework type rags we now use braided tugs with a ball on the end (tennis ball or some sort of rubber ball). If you can start with this now it will also help you later on if you are serious about trying flyball. You want the dog to target *low* so IMO the best toys (especially for larger dogs) are long tugs where you can drag a ball in the runback.
I use whatever toy *I* think is safest/most appropriate. Often when I try a new sport and introduce a new toy my dogs aren't really interested but that's just too bad
What works for me is backtying the dog to a tree or the hitch on my van (attach them with a harness and 15' line) and then just doing prey type work where I'm dancing around, whipping that toy around on the ground, getting them all crazy up for it. Like what some people call "bitework" but to me this is not bitework, just making a dog nuts for the toy. When they are really focused, barking, going nuts for the toy then I pick it up and pop it in their mouth.
This is also a great opportunity for me to reinforce them targeting the ball on the end and not just biting at random (Luna needs this since she has a habit of jumping at her owner and biting her hand or trying to bite the handle instead of the ball). Really I just make the dog crazy and feed them the ball. Also, sometimes the dog grips on better because there is some resistance, being that he is attached to the tree or vehicle.
Using a longer style tug puts some distance between you and the dog making it less confrontational, so you can turn sideways. If your dog has a good grip and is countering, trying to tug you backward, get all dramatic about it. Grunt and let him pull you over and pull the tug away. I get so used to Nikon's extreme desire to tug, fight, and overpower and how confrontational I am with him in play that I take it for granted and have to remind myself when working with a younger or new dog (or anything other than a cocky GSD) that I'm not too confrontational and not unintentionally bullying the dog.