Blossom
I can understand that through wanting to protect your son from this hideous man, you might wash over what he himself has done. But I really do hope that you read him the riot act. He trespassed on someone else's land. He stole someone's stuff. He took drugs for fucks sake, and he's only 14!!!!
That's all really shitty behaviour.
Of course, the farmer is a complete nut job. You definitely need to be reporting this to the police. They take stuff like this pretty seriously. Ask your son to be honest about how much this guy is growing. I don't believe for a second that your son hasn't been back on many occasions, whether or not he's been taking stuff each time. Thing is, if he is only growing one small tunnels worth, then it's probably for personal use, and the police would need to focus more on the threat of bodily harm than the drugs. If he is growing a lot, then he is clearly part of a wider chain, and the police would be VERRRRRRRRY interested in that, and therefore you wouldn't need to focus so much on the threat aspect.
You say someone else's child also got a similar letter?
To be perfectly honest with you, I would go to the police station together with this other family and report what has happened, and I would do it today.
Farmers often have guns, and something like this would get his gun license revoked, not to mention his guns confiscated quicker than you can say "bugger that". If you are worried about consequences to your child then speak to the police about it.
This guy is clearly fucking deranged.