Relationshit

I have been with this guy for 5 years and recently he has become a COMPLETE ass. Not to mention he is acting fishy as fuck. Anything and everything from hiding weed, cigarettes and alcohol throughout our house, frequently changing his lock on his phone and hanging out with "buddies" from work that I have never heard of. Now all of this is new developments. He has never smoked- weed or cigarettes, drank alcohol or has passwords one his phone. I have never gone through his phone, but I feel the need to do so. He is like a stranger now. The last 4 months he has changed tremendously. I need advice and tips on how to handle this and how to sneakily go through his phone behind his back.