There is a reason for age restriction on Videogames. I'm sure your probably aware of the content you're allowing your son to access, graphic violence, sex, horror, drug use etc, but that's one issue. If you want to let your son play online in an 18 rated game he will be mixing with people 18+ who can be quite toxic towards other people online.
I'm sure you know all this and I'm not trying to lecture you.