I think you really should have proof before you make any deep decisions. Perhaps if you catch him stealing or actually find something on him or doing some though I think searching his stuff may be slightly unethical but who am I to judge.
Your situation brings up a deeper question in my mind though and that is the wisdom of dating someone who is also in sobriety? Is it not the point of quitting drugs to remove oneself from a particular lifestyle? I could see how having a mate that may crash you down as being potentially dangerous.