Baxter is also a Boxer/Lab mix. He is 5 and a half months and also eats Purina Puppy Food. I buy him the Puppy Chow for Large Breeds. He doesn't seem to have much trouble keeping his balance, but I have noticed a few times that if he is travelling the stairs too soon after getting up from a nap that it takes a minute for him to really get going.
I'd ask the vet...might just be a growth spurt and he has to let the rest of him catch up to those long legs.