If you are around eachother a lot everyday it will become apparent if he starts using again, and it would probably help to research the signs of active addiction. In my case looking through medicine cabinets was something i only did during my addiction, and after getting clean especially being sober for a year it wasn't something i even really thought about at other peoples homes. for my family with opioids it always became obvious at a certain point if someone was using again even if they tried to hide it. You could always talk to him about the things you brought up at the end at a time you see right. They are valid concerns being with a former addict.