Living with an attitude of gratitude
Here in the US you buy drug testing kits and you can also purchase them online. Just look up drug testing kit.I don't know how I would get him drug tested. That's not a thing that happens here in Australia unless you are arrested, pulled over by police for random testing while driving, or if the person is hospitalized. There may be a way but I have no idea what it is.
From everything you have shared it sounds like you have given much thought and consideration to what you are dealing with.
Even though you have a safe to lock up valuables, unless it's large enough to not move, I would suggest as @Nomad did, to get a safety deposit box at a bank. Especially if he will have people in the house. You just never know what someone will do.
The only other suggestion I would offer is to put up some spy cameras that you can access online. I would put them in your bedroom and in common areas of the home. No need to tell your son that you are doing this. This way, if you see something going on that you are not comfortable with, you can contact someone to come over immediately.
You know your son best and hopefully he will have respect for your home in your absence.