Sorry that i didnt answered your email but i am very busy at the moment.
You wrote that he has an ear infection that can also explain why he snaps at somebody who approch him just because it hurts.
You dont want people touch your head when it hurts.
Diarriah, well from raw to kibble at that age could be the trigger too but parvo is very common also.
We are never ever be able to be that clear with our punishment than his mother or other dogs are.
If you punish hard from dat one you must be getting harder and harder and on the end you will loose that battle from a dog this size.
Be firm and consistent and fair.
I think you have a wonderful dog and with the rigth training and socialisation he will do great.