Personally, my take on things:

Teens are going to talk about, experiment with, get interested in, and what not, about sex at some point. It happens. We don't like to think of it as parents, but teens are sexual beings in their own right, and its totally normal for them to experience sex in different ways.

My biggest concern would be if this girl is extremely sexually agressive. That to me suggests that there may be some issues going on there, perhaps she feels the need to act out sexually. Thats a fine line though - as some people are highly sexed, some not so much. Is there much you can do? Not really, i mean, you can tell your son that hes not to be exchanging sexual messages on the phone you provide, but they will find a way to exchange those messages somehow else, and at that age, messages are the LEAST of your worries