OMG, this is a problem that I would talk to my network and sponsor about if I were you.
You said he has been in recovering for 1 1/2 years with two slips.
I'm assuming he has had the two slips while in recovery so this is not clean time.
If he doesn't have a sponsor, is not going to meetings regularly, is not reading AA or NA literature, is not praying, and is not working the steps and helping others chances are great that even if he didn't use as you suspect he will eventually.
Alcohol and drugs are just the symptom of the real problem. If we don't take action to change we won't change.
I would take time to think over what advice others give you and then pray to your higher power and ask for help in what you should do. Keep in mind that your sobriety is number one so please protect yourself.
I think you have two options, confront him in a very concerned way especially if he has not been working the program.
Or you can let it go. The queston is can you let it go?
If you can't, then you are compromising your serenity. If he is clean, he may be offended but if he loves you and is working the program he will understand. If you let it go. Do you believe that if he was using he will just completely stop again?
I hope that this works out for you no matter what the outcome.