I Want My Son Dad Only to Visit My Son When Im Present

you have your lawyer request only supervised visitation.

You will need proof as to why you don't want your son having visits without you present. That can be difficult to do if you have no proof that he is doing anything wrong.
The girlfriend is a separate issue and you will have to have proof as to why you don't want her there.
I personally don't think either parent should introduce boyfriends or girlfriends too early on as it can be upsetting to the child. So, make sure you aren't doing the same thing before bringing a complaint. If you have proof that girlfriend is dangerous, that's another thing. But just her presence most likely won't be enough to gain supervised visits.
You also want to be careful making it look like you are not willing for father to see the child unless you are present. You are separated/divorced after all and many courts will agree that fathers can handle time with their children unobstructed by an ex.
So, unless you have validated concerns, what you can do is go back to court mediation and discuss these concerns in such a way that it sounds as though you are wanting to work things out for the best interest of your child.
In my case, in California, where I did have valid and substantiated reasons for my daughter not to be alone with her father, a third party was appointed to supervise the visits. That way there was no "he said/she said" and due to the volatility of the situation, we did not have to be in the same place at the same time which was better for our child.
You can ask the courts for relief in conflict resolution.
But, as far as I know, you can't demand that the child only see the father if you are present.

Best wishes.

I feel for you in this. I had to have over night visitation stoped with my ex. I could not do it thorugh the courts because their answer was "they can't force him to be a better parent." I just refused to allow over nights, I knew he could not take me back to court to fight it.
What would happen if you offered to let him see your son at McDonalds with you there to watch?
Is there any one thing that you don't like about his girlfriend?

I would think the only way to do that would be to get an attorney and go to court to change the visitation agreement. You'll probably have to have some back up as to why you think they way you do. An attorney would be able to tell you if those are valid reasons or not.

Unfortunately, you don't have the ability to demand that unless there is proof of her having hurt your son in the past. Typically, when there is court ordered supervised visitation, there has to be CAUSE and the supervisor isn't YOU, it is a 3rd party that is neutral.

Why don't you trust his girlfriend? If you have reasons, present them in court is your only chance. If it's just because you don't like her, that is not a reason.