It is hard when your children move off and away and onto lives you don't know much about (I mean you don't know what they are getting up to, not about things like sexuality). It's actually lovely your son has met someone, but don't go crazy keen on getting him to bring his boyfriend home, he might have not even decided he's the right one for him yet and he just needs time for the relationship to bed in (or meet more people). That's for a later stage. He obviously does feel secure with you if he's told you about his new boyfriend so ignore people trying to make out you are not handling this well enough, all sounds very positive to me, just don't go overboard just yet with the mum enthusiasm.
As for the drugs, I know teens and whilst not all do drugs, a heck of a lot do. I would actually be reassured he's engaging in harm reduction and doing this testing, it may have been encouraged at his Student Union, they may have suggested it and he's followed their advice. Drug testing is increasingly common at festivals as well. I would always advocate a harm reduction approach, just saying don't do it doesn't work for those already taking things. The only thing is I wasn't aware of huge amounts of mixing of fentanyl with other drugs in the UK, certainly not outside heroin, possible cocaine? perhaps MDMA but I am not on the ground and don't know the real-world risk right now, it's higher in the States. Anyway, I talk honestly with my children about drugs, and have told one of them due to their MH conditions I wouldn't advise taking them, the other one has experimented but does very little now due to a bad experience, I think don't act shocked or ban anything, but it's fine to look on websites like Frank and talk about the risks,just keep the lines of communication open, you will find that at nearly 20, it's not something you can police except in your own home.