13 can be a bit young, but i wouldn't ever discourage smoking some weed. Explain how you feel about it and if he stops then its all good but if he doesn't, don't hold it against him, smoking pot is great, its an all natural drug that makes you feel great. it makes tons of things better. your mind becomes more creative with what ever your doing, if its listening to music or watching a 3d movie or just watching t.v, even physical activities are sometimes better, I use to get high and go skateboarding all the time. If he is just doing to because he thinks its cool or because he thinks other will think he is cool then i would say to stop smoking it, people like that are whats keeping pot criminalized, Im going to be a married man is 8 month and i smoke weed every single day for the last 5 years. I live a very normal life in a great house with a great career, great goals and a very loving beautiful wife. Im just an everyday normal guy and I smoke weed by myself at home.
Like i said, 13 is defiantly too young but don't judge him on just smoking weed, judge him on his personality and choices he makes while under the influence. because if the he cant take control of situations while high then he doesn't deserve to smoke it, and like i said, people like that are the reason weed is given a bad name in the eyes of the law.