As I read all this it reminds me that my second son used to bite my upper arm at that age (I think I blocked it out). If I were holding him and talking to someone, it was as if he were bored and wanted me to himself, he would just take a tiny bite of that fleshy part. There is no way to -not react- to that! OUCH!
I even quizzed his doctor. He said 'my sister works for child services, therefore I can not tell you to spank him.'; 'However, you need to 'shock' him.' I didn't know what he meant by that, I decided he meant to spank him. Since I was holding him, I started giving him a quick smack on the big part of his theigh. It did stop him, or he grew out of it around that time. I was always very calculated whenever I spanked my children, the very few times I did; Never swat randomly at a child.