Help please When to start setting boundaries

Our infant is now 11 months old he was a premi so became a sahm and i spoiled him bad did everything for him let him do what ever he wonted it was cute then but now he is starting to hit and bite and throwing things not letting me put diapers on him when he needs changing i wonted to start telling him no and gently popping his hand but i feel he is to young at what age and or what punishment did you use ? When i tell him no or pop him he looks at me like i broke his heart and starts crying and i feel so bad do i buck up and set rules now or wait longer