Hi! I'm just south of you. You are a wonderful person to take on this little boy. I would wait to have a blood test to determine the best food for him. I feed nat bal, too. Mine didn't like the nat bal meat roll very much plus it was very hard to cut - at least, for me it was!
I mix the nat bal with blue buffalo. I bake chicken breasts and feed half & half. Mine loves it. I also give her a snack at night of blue buffalo canned chicken & veggies. She really likes it and it has worked out very well for us. I'm curious what the cough is that your lil one has. I hope it isn't collapsing trachea. I would get him to the your own vet quickly, so you can start him on the rest of his life. I'm sure he will do better from now on. The wobbling is worrisome and I hope he doesn't have any neurological issues or liver issues. Please keep us updated and I send you my very best wishes.