My son has been hanging out with Muslims and I'm afraid he'll be influenced by Islam.
Why don't you want your son to be influenced by Islam? What part are you concerned about?
It seems like a religion that's very oppressive towards women and I don't approve of that.
I know you have negative feelings about how women are treated in the Islamic religion but you should keep an open mind and learn more about it. It's wrong to prevent your son from joining a religious group that you don't approve of. You should learn more about the religion before being so negative towards it.
I don't need to and I think it's bad since it preaches terrorism.
That is an incorrect stereotype. The vast majority of Muslims have nothing to do with terrorism and are great people. Get to know the people your son is hanging out with, so you see that this just isn't accurate.