Am i being unreasonable???

To feel so upset over my ex keep bringing his girlfriend to see our son when he takes him out. He only sees him for 6 hours on a sunday, 2 of them he is having his nap so 4 hours in total, and he has to have her there? I could understand if he had him for like half the week, but half a day? Its driving me mad, i just feel bad on my boy that he isnt getting quality time with his dad, and hes gonna get confused, and im worried he might get attached to her then he dumps her aswell? Oh i dont no, maybe im just being insecure about it all probs am. I did mention this to his dad and he totally raged at me and said he 'can do what the f@@k he wants' am i being unreasonable? Hes been with his girlfriend about 6 months now is that enough time for them to be seeing each other? Any advice would be really appreciated. I no if it was the other way round i would not have any man near my boy unless it had been a long time together as i have seen how hard it is when they walk out, seeing my daughter how upset shes been with my ex just leaving has made me not want that for my son. Xxx