op, your best bet would be to talk to
\link{http://www.ceop.police.uk/\ceop}
they will be able to advise you as to what course of action they may be able to take.
Queengigantor while what you are saying wrt being in a relationship is correct (given actual intercourse has not taken place), there are laws about grooming underage children online, and if there have been inappropriate conversations between the op's ds and this woman (of a sexual nature) and the woman knew he was underage (as seems t have been implied here) then she can be brought to account for that. But unfortunately it's not likely to be an easy process and op's ds will almost certainly have to be interviewed in depth by a child protection expert, computers removed for analysis etc in order that a potential prosicution be brought.
I would also go and speak to your ds' school. Because the likelyhood is that if he's met this woman on a website, there are almost certainly others from his peer group who are visiting it and who might have encoutered other individuals with potentially equally questionable motives, or even that some of them are lying about their ages and have been talking to people who have no idea... an internet safety talk in school would not go amiss, and the school will have links to organisations who can do this sensitively and appropriately for these children's ages.
I think that all too often we are complacent about children on the internet, often because we ourselves don't have the knowledge to keep them 100% safe, and also because technology holds no fear for children, and a lot of them are more IT savvy than their parents. But sadly the internet also gives people a false sense of security and they open up more than they perhaps would in rl and potentially take bigger risks. It happens even with adults, so with young naive impressionable children the risks are tenfold.
I attended an internet safety talk recently and was shocked to be told that in a recent survey 15% of children between the ages of 11/16 admitted that they had met up with someone they had met online, without their parents' knowledge. Another 10% said that they had taken a friend, but still that no adult knew. So essentially 25% of children have met up with someone in rl that they've met online, and their parents or any other adult were unaware. That is a chilling statistic...
Op - you need to clamp down hard on your ds' internet use tbh. 13 really is too young to be having a facebook page, (regardless of what the site rules say), so I would delete that for starters, keep his laptop in a family room where you can view it at all times, investigate getting some parental control software so that you can keep a better view of what he's doing online.
I do not subscribe to the view that children deserve privacy online - as parents it is our responsibility to keep our children safe, and clearly in this instance the op's ds has shown that he is not responsible enough to be given unsupervised access to the internet.
Good luck.