I smoke with every member of my family that smokes haha. Im twenty and ive been smoking for 5 years, But it wasnt until about 18 that I actually let me family know that i was smoking. Now fast foward a few years and i now smoke with 5 of my cousins, my mom, my dad, my uncle(med patient in cali,best to smoke with haha.) And more im sure im forgetting. So in all honesty if you say HE wants to smoke pot, he's gonna do it. So IMO i say you might as well be the one he does it with. Atleast you know that the weed you smoke with him isnt fucked with in anyway, and you can make sure he doesnt get too high and not like the experience.
If he doesnt like it then leave it at that, but if he does maybe teach him everything you know so he doesnt get his head filled with all the Propaganda that can come up when talking about weed from an uneducated person.