OP, I live in the Netherlands and have had to get used to quite a different attitude towards weed. Can't stand the stuff personally but here it is seen as a choice you make and you can also grow a bit for personal use. People pop out at lunch to have a joint and then head back to work again which made my mind boggle a bit but they were perfectly normal, functioning adults who liked to smoke pot once in a while. There are many moves afoot to regulate the amount of THC and heaven help you if you are caught growing more than your allowance. Ten thousand euro fine minimum and you hear helicopters all the time flying around looking for weed farms.
It doesn't carry the same stigma as in the UK, and it's of course heavily regulated and you have to be over 18, but I still can't help but cringe at the smell whether it's walking past a coffee shop or in the park or street (illegal to smoke in public but there you go).
I hate all drugs due to very bad experiences as a teenager that ultimately led to a friend's death. But as you say, your son is 16 going on 17 and rapidly nearing the stage where he will be leaving home and entering full blown adulthood. I thought it showed that your relationship had strength that he can discuss this with you. He has to reach that decision for himself but it's really positive that you haven't alienated him and he seems to trust you.
There is experimentation with drugs which I think most people grow out of or become habitual pot smokers like my aunt and uncle who work and are perfectly fine, and then there is systematic abuse over a period of years that leads one down a road of harder and harder stuff until it's too late. I'm sure you son is in the former category and it also made me think, remembering what happened to some of my friends, it could be worse. If weed is as far as he wants to go and he's rejected other kinds of drug then thank god for that. Hopefully he will stop.
What he chooses to do outside of your home and what goes on in it are two very different things. No way should you allow weed to be grown - not only is it illegal but that in some way shows you kind of condone his behaviour. And it smells terrible. I get that there is some seriously altered pot on the market, stuffed full of chemicals and that home grown is better perhaps, but it's against the law and you do not want drug paraphernalia in the house, end of story. Your son needs to respect that. Your house, your rules.
Just to add to a comment about weed causing certain mental illness - it can be an early trigger to something that is already there i.e. there is a genetic predisposition that you were going to get it anyway and a life event could trigger it through to drug use. I have bipolar and I would have developed it no matter what and it was not down to the very small amounts of weed I smoked as a teen, but was an inherited illness. I think an earlier poster was saying it just means the onset is faster than it would have been otherwise in some cases.
Good luck 