Need advice about my dogs anxiety and aggression!

I have a 6 yr old border collie mix who is sweet at pie but has gotten more aggressive since my husband and I married a year and a half ago. My husband has a son 6 years old, who has caused major anxiety for my dog. He has not been kind to him in the past, plays rough and actually he sort of gives me anxiety too. 😳 How can I help my dog? I hate to see him become aggressive bc of this one child.