S.D.
Why are you using formula at this point? Did the doc say you needed to? You can put kids on cow/soy/goat milk at their first birthday (saving you TONS of money over continuing to buy formula) and you won't have formula poo any more. You mentioned the bananas which can be allergenic - more so as they ripen up. As long as he isn't constipated or having hard poos, don't worry too much.
Good Luck!