My nephew sucks on his arms out of anger?

My nephew just turned ten. He has had a rough life and a horrible upbringing with no father but multiple "guys in his life" and a mother who wouldn't be given the okay to care for a pet rat. My mother has temporary custody of him now (Thank goodness)!! The problem is when he gets told NO, he runs off and sucks on his upper arm to the point of causing a hickey. He has told us he does this because he's angry. We have asked his doctor and they say it's just a tick but we are concerned he may be at risk to cutting or other self mutilations in the future. Any advice would be greatly appreciated. Thanks!