I'm sorry to sound negative, but the likelihood of your vet knowing enough about canine nutrition (particularly large breed growth) isn't very good. I would maybe shoot some xrays if he were my dog, see how growth plates are looking from elbow to "wrist" (be sure to get good view of distal physis), as that will give an idea of how much better/worse he could possibly get. If he has one bone growing and one not, there are procedures that can be done to straighten those feet. Excessive East/West feet can put strain on other joints, and one bone that's stopped growing will make his elbows not fit right and his anconeal process may not unite. But beyond xrays to tell you what the joints look like (and many vets aren't even good at that...) I don't think you're going to get much help from your vet, unfortunately.
A proper diet does NOT need supplements, in puppyhood, at least not minerals. If you want to supplement, stick with digestive enzymes, probiotics, maybe some kelp-type additives, etc.
I have no idea what Trixx means about his tail, either.
Here are a couple links explaining findings about harmful supplements.