Parttime, you are at risk of creating a bigger problem here, while you try to 'solve' a small one.
Those drug test kits are only any good as a preventative measure - and they are not very good even at that, since many kids (your DS included) smoke/take cannabis even if they know their parents have the kits in their drawers. Once a young person starts using, the kits are absolutely no good at all, because they give you the info that your child has used cannabis but nothing more... What you really need is info about the drug's risks, or what to do about it...
The point is, teenagers will take drugs if they want to. Parents cannot stop them - they can only inform and help to motivate young people to make positive choices.
You need to ask yourself what you want to achieve here. Presumably, most of all, you want your DS to be safe.
To be safe, your DS needs good, reliable info. He needs to know about the drugs he's taking - their effects and especially their risks. He is more likely to get this, if you make yourself better informed too - start here: www.talktofrank.com/
To be safe, your DS needs to be able to talk to you, and especially to tell you if he has a problem. If you make drugs taboo and do random test on him, he will stop talking to you.
Just to give you an example of how important communication can be... I had a scary experience with my DS in the summer, when he took something unknown, and the friend who had given it to him started having seizures and an ambulance had to be called. Because my DS trusts me, he told me, and I was able to then take practical steps: e.g. make sure he made himself sick and drank lots of water; got the name of the tablet he'd taken so I could google it, and discover if it was likely to be a medical emergency; watch him while he slept; have a conversation with him the next day about how very stupid it is to take unknown drugs... Ask yourself whether your DS would be likely to talk to you under similar circumstances? If not, you are not going to be able to help keep him safe...
To be safe, your son needs to know how to evaluate and manage risks for himself, and make his own judgements about dangers. This may well include trying out some drugs you would much rather he did not try, but it is all part of growing up. As you say, he is off to uni soon, and he needs to be able to keep himself safe by then. If you drug test him now, you risk two things: (a) he won't need to make so many decisions for himself, because you are keeping more of the control, and (b) he may just postpone experimentation until you are not there to help him...
To be safe, your DS is probably better off smoking weed than taking many other drugs. Random drug testing for cannabis may actually make him less safe, because he may switch to other drugs that cannot (yet) be reliably tested, like m-cat, or ones that have a much shorter 'test life' that reduce his chances of getting caught unless you are testing very frequently (e.g. cocaine and I think speed are only in your system for 2 days). I can imagine a situation where he has drugs 'binge' nights after you test him, because he knows he'll be 'clean' again by the time you next test him :( Incidentally, I do not think cannabis is harmless, but many other drugs are worse. Skunk is bad, and imo should not be considered the same as other forms of weed, because it has different and much worse effects, including a risk of psychosis.
To be safe and happy, your son needs to be able to continue his life, including studies, work, relationships, etc. It sounds like he is already doing this. For as long as he manages to do well at college/uni, and engage fully in life, I do not think you need to worry to much about his weed smoking. Lots of 17yo's try weed, and 30% or so smoke it regularly, but only a small proportion develop any kind of problem with it. IMO there are two entirely different types of use: smoking to relax at the end of a busy day, and smoking to block out life. If you keep communication with you DS open and honest, you are much more likely to spot any warning signs that he might be slipping from one group to the other...
To be safe, your son needs to be motivated to have a full happy life. Young people who have aspirations and things they enjoy in life do not tend to wreck their lives with drugs. They may take them, but they keep their use under control. If you feel your son does not already have this (though it sounds from what you say that he does :) ) then you can help by supporting him to find things he really wants to do - things that will engage him and give him joy and satisfaction - whether that is useful work or new hobbies. :)
It will probably also help you to talk to him and understand why he takes drugs, how well informed he is, his understanding of risks and 'risk management', etc. My advice, seriously, would be to throw away the drugs test kits, and start talking.
Good luck!