I'm not sure about what legal issues he would face, but look at the emotional side of it.
They are only five years apart, it's not a world of difference, the only prolem is that they are young and when your young five yearscan seem alot of emotional change. If the 18 year old was 29 and the 13 year old was 24, would you still see this as a problem?
Strange things happen, believe it or not and however weird it might sound to you or anyone else, they could accually be in love.
I know that this happens, from experience, me being the younger out of the two, and when you get to know someone age doesn't come into it.

Why on earth is your 18 year old dating a 13 year old in the first place???

Tell him to end the relationship and have absolutely nothing to do with the 13 year old, otherwise he could face jail time along with being forced to register as a sex offender, which will haunt him for the rest of his life. He can say goodbye to jobs, financial aid and future relationships.

Then tell him to grow up and date girls his own age.

Can w say jail time, sex offender regestration?

you should get him into therapy, immediately (you might need it too, since you are only worried about the legal ramifications of you son doing a 13 year old.) do not pass go, do not collect $200.
a "normal" 18 year old would not have anything in common with a 13 year old and no reason to form a relationship, sexual or otherwise, with one. he is either a predator or emotionally unformed and you should, as a responsible parent, GET HIM HELP!!!!!!
It's not ethical and someone that young is not capable of making those decisions. I hope he gets busted for rape, because a 13 year old is not capable of making adult decisions like that, and you know about it, so you are guilty too.

Guest11611067 = Fail
Guest11778426 = Fail
Guest12155551 = WTF M8?
Look heres the real story. If girl is 13 and boy is 18, its gewd coz she gets a proper penis. If the boy is 13 and the girl is 18, wtf is that bitch doing? Shit happens, but this is not it.

It depends on what state you are in.

F* the 18 year old hes a perv

Going to jail, the oldest you could be is three years above whoever you're dating or else it's illegal.