The tree will either straighten itself out or not. I know that sounds simplistic, but a sandy to sandy loam soil with sharp drainage is what your tree wants, so there's a good chance it might not be able to deal with the limited oxygen supply in a clay soil. If the tree was floppy when you received it, it likely had issues then. You might take that up with the seller.
I'm not sure that what type of fertilizer you used, or didn't, is at issue, though it might have been while it was still at the nursery. If it was at issue, it's effect's would be made manifest in the new growth, which appears to be adhering to the plant's normal growth habit, which should be seen as encouraging.
Even though the lower branches are laying over, you shouldn't feel as though that will permanently detract from the plant's eye appeal. If it can deal with where it's planted, it should end up looking fine in a couple of years.
Once you're fairly confident the tree will remain in a vertical orientation, remove the tether. At that point, you can prune the lateral growing branches back to a secondary branch growing upright from the top side of the primary branch you'll be pruning. Prune at the point that best maintains the plant's natural taper. You'll know by the end of this summer if the tree is likely to realize a large enough measure of its genetic potential to be something you want to keep.
Al
