Honestly, the foods you listed that you fed him, aren't that great
I hated Nutro Ultra my dogs would poop orange..
My adult dogs (and by the time they are 8.5 months) are getting 2 cups twice a day.
I hate to say it, but I'd try a different food and give it a good two months.
There are sooooo many foods to choose from, what works for one may not for another.
I think exercise can bring on soft poops as well..I keep mine toned down an hour before I feed and a good hour after (sometimes longer after).
Have you run fecal testing on him? sorry if missed that,,as in giardia testing?
He might need a probiotic, Masi can have soft poops off and on, I have been giving her Opti Gest(?) I get it at petco, it's relatively inexpensive and has calmed her gut down ALOT, I do 1/4 tsp each meal (powder form)..
I feed Diamond Naturals, have fed it for years, tried other brands always tend to go back to this, my dogs do well on it and it's not that expensive.