I think that some teens start experimenting at about this age. You need to have a very frank and open talk with him. Let him know that sex is not a game. It has so many consequences and lots of them involve really strong feelings.
Let him know his feelings are very normal. That you are sure he really does care about his girl friend and the "love" that he feels for her IS strong, but explain that believe it or not, when you REALLY find the right person, that love is going to feel a 100 times stronger than it does right now.
Let him know that if he has sex with every girl that he has such strong feelings for, it could end up being a lot of people and it makes sex not as important or special as it should be.
Also let him know there can be lots of regrets for both of them later, because feelings can change. He does not want her to have regrets and you are sure she will not want him to have regrets.
Also have him tell you the possible consequences of having sex, so that you can make sure he really understands them.
I have known my husband since we were 13. We were not sexually active because we just did not think about it.. I do remember when we were at the end of our freshman year in high school. His father gave him a CASE of condoms! We were so puzzled. We laughed and said, well boy are they on the wrong track.. At my house my mom talked about the consequences and examples of people we knew that had babies at very young ages and other the consequences so much and was so open with me and my friends.. It was just not something we were interested in.. It sounded like a minefield.