How will I stop him doing this without causing family problems?

I am assuming you live with your uncle since you havent stopped letting your son be around him. I don't know how old you are but if you are old enough to move out, I definitely would because this behavior towards your son is unacceptable. Your uncle needs some serious help and you need to get your son away from him because he might seriously hurt him one day. And with this new baby coming along you dont want to put it through this either. As far as the "causing family problems" part of your question, if your family really cared about you and your son, they should be on your side about this situation and would want you two away from him. Does your uncle act different towards your son when he is around your other family? If he acts the same way, I don't see why any of your other family members haven't said anything to him yet. I know my family would have defintely said something to my uncle if he treated my daughter that way.