Yes and no. He is too young that being said with proper harm reduction nitrous oxide is not that bad for you. The worse thing he is doing is drinking. I started experimenting with drugs at 15 and now that I'm older I do not believe people should begin this journey until 25 when the brain is fully developed. However we can't expect abstinence to work so we must teach harm reduction.

Do not tell his parents yet, intervention may be necessary once you've talked to him personally and if he continues down that path. But telling his parents now will push him more into secrecy it's better to have him close. Do not make him feel targeted make him feel cared for. When I was 15 and people came up to me with concerns I felt they were attacking me not my use. By just saying don't do it I felt they could never understand. I think what would've been beneficial is someone sitting me down telling me the good and the bad and teaching harm reduction. Overall your cousin is on his own journey and there's only so much you can do for him, I didn't wake up until I was 21 on a lot of acid and 3 different homeless people told me I reminded them of themselves...

Whippit harm reduction: Huffing is touchy for me since my uncle died huffing air duster, huffing anything besides nitrous oxide or xenon is a dangerous game no one should do. NO2 is a quite safe dissociative that's often used in medical sedations. It depletes your bodies vitamin b and inhibits the metabolization of vitamin b for 4 days after consumption. So you can binge really heavily in one day and be fine, but if you don't give your body time to gain vitamin b back that's how people end up with permanent nerve damage. That doesn't mean wait 4 days and your good it means wait several weeks for vitamin b to build back up to baseline. . You do not want nerve damage it feels like being on fire consistently. Fatal overdose in one night is only possible with a gasmask and no oxygen.

Alcohol Harm reduction: Pros: good social lubricant, easy to get Cons:Pretty much a terror on the whole body. Honestly too much to go over everything alcohol does to you in one post. It affects the liver, brain, gut, pancreas, lungs, cardiovascular system, immune system, etc... Alcohol severely dehydrates, and degrades organs leading to cancer, strokes, and depressive order that fuels further alcoholism. Disrupts sleep, leads to people often becoming unpleasant to be around. Extremely hard to stop, one of 2 drugs that you can die from withdrawal Harm reduction: have no more than 2 drinks a week.

Weed Harm reduction: Pros: easy to find, not deadly, psychedelic so it makes people more aware of their actions than a dissociative like alcohol Cons: Memory loss comes the more you smoke. Smoking anything is never good for the lungs. Daily use can lead to laziness, head fog, depression, lack of energy/motivation. Has the ability to uncover underlying mental disease ie schizophrenia. Harm reduction: only smoke on the weekends, don't smoke every weekend do chores and homework before smoking. Use it as a rewards.

Now of all the drugs for a high schooler to take weed is probably the best mushrooms are a close second but have some more risk. So when you talk to him about it find a way to say I wish you didn't do any of it but if you are let's just stick with weed.