I started when I was fifteen. I'm fine, I'm on my way to university to do psychology! In fact, it probably allowed me to do better at school and made me happier in general and let's just say it held a lot of benefits for me with basically zero adverse effects. Pot is mostly harmless. I know that probably doesn't mean anything to an 'anti pot' mother but just consider this, would you rather: a) your son go out and get totally drunk off his face, do silly things out of his control, make a fool of himself and poison himself sick? or b) smoke a bit of weed, relax and chill out and watch some t.v. with mates?
Keep in mind that that is not an indication it will happen like that for everyone. I for one am a weed-addict. I tried A LOT of different illegal substances (the ones you can try out without risking your entire life, if you are careful, so the ones not physically addictive) and I have to say for me weed is the one most difficult to stay away from. I never had any addiction problems besides weed and I tried a lot of substances. To give you a picture: MDMA, MDA, LSD, DXM, DMT, 2-CB, 2-CI, psilocybin, salvia, peyote, LSA, nitrous, truffles, coideïne, tramadol (these last two are physically addictive but I only used them a few times to know what they felt like) and I'm sure there's a few that don't come to mind right now, but the only problem I had/have is weed. It's because weed gives you no feedback like other (hard)drugs do when abused. You won't feel bad if you use it all day every day, you won't get a comedown, you won't get withdrawal and you won't get psychological problems. Those are things that only surface after years and years of abusing the substance. Like a lot of people said before me weed is pretty harmless when used in moderation but in my experience (and I am far from the only one) in the long run it will for some people turn to an addiction that is VERY hard to get rid of and after years will influence a lot of aspects of your life.
To put things into perspective a bit: I too am almost done with my college education, I too have a girlfriend and a lot of friends, I too am very active in sports and am very sociable. But the addiction is there. If I can't smoke I will start to lash out at everyone in my immediate surroundings, I will develop headaches, I will only think of how to obtain my next high (which is like going to the store in the country I live in) and I will generally become a weed-fiend

. Depression follows the first 5 days without smoking and after that almost all influence of the substance is gone, but the psychological addiction is still there. I managed to quit for about 8 months recently but I had one little slip-up and am now smoking every day again
So all in all of all the substances he could be using (even alcohol) weed is the least dangerous. And probably it will not cause any troubles for your son (though he started very young, I was 13 too when I lit my first joint, I am 25 now. Looking back that was way too young and probably why I am addicted now). But do not let ANYONE ever tell you that it is completely harmless and not addictive because this is simply NOT true. With years and years of abuse addiction will come and it is a very hard addiction to beat. So my advice: let him experiment, though try to give him the complete picture, not just what his friends say because they are young and they do not know anything yet, not about drugs and not about the way the drug world works (like a few already said if you do weed you will eventually come into contact with other drugs too). Try to keep an eye on him (you will notice when he starts abusing it in the long run). Weed is not something you should be demonizing (which seems to be what my reply is doing, I am just showing you an extreme example of what weed can do in the worst of circumstances so you know every possible outcome and can calculate it in your actions) but it is also not something you should take very lightly. Keep an eye on him and he'll probably be fine though, there are a lot of worse things in this world, I for one am grateful it is just weed I am addicted to and not something else, it is holding me back A LOT but it is not destroying my life, my love or my family. I am sooooo glad I always had the smarts to say "no" to things that could potentially destroy my life (never did speed, coke, meth, crack or any other very addictive substance and I never will, the risk is far too great for me because I have a personality prone to addiction) but that is because I educate myself completely before I consider trying something new so I always know what the risks are beforehand.
*edit* Oh yeah one more thing: I would've pounded the man that gives my son drugs into dust or I would've reported him to the police (though that would take a father away from his child). I really CAN NOT understand someone who is right in the head giving a 13-year old drugs, just hearing it happened to someone I don't know makes me mad, where does he think he gets the right to do that? He made a decision for your child that only your child or you (if that's even possible) should be making. He just decided to turn your child from drug-free to not drug-free anymore at an age where your child isn't even able to make such a decision for himself because he is not yet fully developed and does not yet understand the consequences...
*edit 2* Something I forgot are the legal ramifications. For me they are almost non-existent. I do not live in the Netherlands where it is legal, it's still illegal here but barely. To give you an example I was recently (for the first time in my life) caught with 5g of dutch cannabis (not a small amount, but not a garbage bag full either) and they just confiscated it and I had to be questioned about where I got it and such. I didn't even get a fine or anything, just a warning. The police here are very liberal but maybe that's not the case where you live so that's another thing to think about.
Sorry for the long text but I wanted to give you a complete picture of how it is for me and how it is when it doesn't turn out like it turned out for 90% of people that smoke cannabis...