It's not the allergy. Casein allergies are VERY rare and the symptoms are as follows:
As found here:
Milk Allergy & Intolerance Note that none of the allergies mention discipline problems, they are all physical in nature, not mental.
Sounds like the terrible twos to me.
I can't believe you actually slapped your 2 year old across the face!!! That just horrifies me.
Pick him up, put him in his room, and leave him there until he is in a better mood. Never ever attempt to discipline a child when you are not in control of yourself.
And you really think you are ready to adopt another child? Girl, you can't control the one you have already.