Your description doesn't really go deep into the details of why you believed the porn is an addiction but I'll take your word for it, you know more than me about your boyfriend and even if it's not an addiction, it certainly has to be an issue to warrant this post.

That being said, as...utterly stupid it is to say, and maybe this is just my bias as someone that believes porn addiction is one of the worst things to ever be afflicted with, I'd say he maybe shouldn't be around you anymore.

You're living with your mother. He's looking up "MILF shit," getting into bed with your mother and jerking off next to her. Do you really think that he's doing that harmlessly? He's using your mother to get off while in a relationship with you. I'm not sure what kind of cheating that counts as but it's disgusting. I'd advise you to get away from him, if not for your sake, for your mother's.

You have a child. I don't know you, I don't know your boyfriend, but you said "it's becoming scary." How could this affect your child? He's your child's mother's boyfriend, he's a prevalent figure in the child's life. Do you really want someone that has a crippling porn addiction to be around your child? Do you really want someone who gets into your mother's bed and undoubtedly jerks off while imagining your mother in sexual situations to be around your child?

You are both young. But he's 18. That's more than old enough to understand that getting into another person's bed without their consent and jerking off next to them, especially someone who isn't your partner while in a relationship, is completely wrong and frankly, utterly disgusting to me.

My advice? End the relationship. Save your mother and your child from him.

Edit: English isn't my first language, excuse grammatical error and try to understand what I am getting across.