Yes this is an issue that is very hard for us as parents. But I think it is a good idea to be open with him and let him know he can come to you if he needs advice on this subject.
We've had to deal with the issue of our 13 year old daughter, having her weekly or monthly crushes on different boys, some more serious than others. Although I feel she is too young, we've had the long talks with her, about dating, sex, the pressure that can be put on her.
We have given her certain guidelines, we haven't been totally close minded (like the jw mindset) and it has worked for us so far.
I am believe that if you tell a child no absolutely not, they are going to do it.
Give them a choice, they can act on it or not.
cj