As someone who has been in the situation your son is in as a child, please please please do not let your FIL continue to do what he is doing. My grandfather was that way about everything, and honestly I wish my parents would have not let him around me. It sound's so harsh, but better to not have him in his life at all then him feel inferior and worthless. I was verbally and physically abused by my father growing up, and verbally abused by my grandfather, and I can tell you that being verbally abused (which is what your FIL is doing) leaves a much bigger impact and scar than physical abuse does. The bruises and the cuts heal, but I will never forget being curled up in a ball on the bathroom floor (and sleeping there too) while my father yelled at me saying obscene things and making me feel worthless (the bathroom was the only room in the house with a door with a lock). Even after he got better the little comments and snide remarks here and there cut deep. It was compounded by the fact that my grandfather was the same way. A child's parents and grandparents are supposed to be the people a child can count on to support them no matter what. I am glad you are taking steps, but don't be afraid to do whatever it takes to protect your son.
Sully ~Sullivan's Fly Supply~ [17.1 TB] RIP 2/24/14
Rio ~Camperio~ [18.0 Oldenburg]