advice from stepmoms (UPDATE)

This is for ladies who have step children in there life . I have a step son and he is 5. I am with his dad and it bothers me that when I touch his dad either if its horse playing or affection he tells me to not touch his dad or he will scream my name and say stop it or try to hit me . It upsets me because before he never did this but now he does and now we're expecting soon and I don't want his son to say that's my dad to his sibling which I know will happen. How can I possibly explain to him not to say that ?we're all family and sometimes it does hurt when he says that because he's all about his dad and doesn't care much about me (UPDATE) His father will usually laugh then tell him to stop or he will say dads okay. we've gotten into a argument once because I said he needs to stop saying that and my BF was like he's just making sure his dad is okay and I said okay how would you feel if I taught our child to tell you that and he got all pissy. I don't know if getting down to his level will even work just because he's delayed on learning . He acts more like a baby so I'm not sure if he will understand or what way to say things to where he understands.