A: It sounds like you’re in a tough situation because you want your son to have time with his grandfather, but you also don’t want your son to get hurt either. One way you can handle this awkward situation is to say, “Dad, I know you’re busy with your girlfriend’s kids and adding one more kid to the mix makes things a little crazy. I think I’ll come along the next time my son visits with you so I can be another set of adult hands.” This is a polite way of conveying that you are aware of how chaotic things can become with his girlfriend’s son and are offering another adult supervisor.
You may also want to offer your father time when he can come and visit you in your home so that he can have some quality time with your son alone. If your father wants to have his grandson all to himself, you may want offer to leave the house and run some errands and enjoy the ‘adult’ time
Hope you are able to respectfully achieve your goal of grandfather-grandson time in a way that is comfortable for you also!