As long as he's growing, gaining weight appropriately, and thriving otherwise, I would maybe just have your vet check a few serial stool samples even though he's been dewormed... sometimes it takes more than one to find parasites because they aren't laying eggs 24/7, so not all random samples have eggs in them.
Otherwise I personally wouldn't worry too much at this stage. Squash was the same way, he had softish stools for a long time and I've seen a lot of other puppies that just have this sort of chronically a little bit soft stool despite good diet, food changes, deworming, negative fecals, etc. Most of them eventually grow out of it. As you probably already know, there's a lot of immune tissue and various populations of organisms in the gut, and I think for whatever reason it takes some of these puppies a while for their system to get the balance of things quite right as they're maturing.