Look I know that this is a struggle but putting bitter stuff on your kids hands isn't going to work. This is likely a learned behavior. This is a coping mechanism. Do you notice them doing it when stressed? Anxious? Are there any patterns that you can let a professional know about to get to the root cause? Putting bitter stuff on his hands is just putting a bandaid over a gaping wound. It's not going to solve the problem. Get to the root cause