it seems like a pretty intense relationship to me. Its likely that they already hav had sex or are going to really soon. My advice (i'm young) is that you don't read the phone messages anymore because if you betray his trust and things go badly he wont come to you for help. It would be important that your son knows about safe sex and how to get condoms sti and pregnancy are bigger problems than having sex. To make sure he knows maybe you should give him some info or get his dad to have a chat about it. Don't make a big deal out of it and don't tell him you saw the messages. Becaus it would be embarrassing and betraying. Try not to freak out about it. It is normal part of growing ur. You probably stil see him as younger that what he is but if you trust him you will still have a good relationship.good luck