My son is AD/HD and has a girlfriend

My 16 yr old AD/HD mildly mentally retarded son has a girlfriend. They talk all they time. They are in class at school plus they talk all day and til late night. I think I made a mistake of buying him a cellphone for christmas. He talks to her late at night. He sneaks down to his room and talks to her and then tells us he hasn't talked to her. The other night he went off. We confronted him about him talking and spending too much time with the girl for his age. He got up left the room and yelled we think he's thinking of raping someone or something. He goes overboard on things. He's been telling the girl he loves her but she tells him she loves him too. I think the parents are pushing her off on him. They want their daughter to have a boyfriend. She's overweight and mentally retarded herself. I just don't know how to handle the obsessiveness with the sex and the girl. We took his cellphone away. I'm not sure if we should give it back or if he's to mentally irresponsible to have one. He says he's told the girl he can't have all the calls and such. He also says he's not interested in sex or anything. I think he just wants a girlfriend. Just the attention. I don't know. Handling the sex issue with a mentally challenged child has to be different then handling the issue with a child who isn't. I don't know. I have not raised a teen boy. He is my first and I would really appreciate some advice from anyone.