You have a 15 year old son who is in rehab so admittedly has abused drugs. He lives in your home when not at rehab. You are responsible for your son and responsible for drugs in your home, if that happens. His poor choices has put you in this position.
From where I sit, all the above considered would entitle you to check whatever you want to protect yourself and your home.
That said, it might be fair to make this clear to him. Also, if what you find checking upsets you, you might want to give thought to why. Is it normal teenage "stuff", or is it something dangerous to you, your son or others because of his drug use.
Only you can decide what is right for you. As a mama, I know how much we fear for our addicted children whether 15 years old or 40.
Keeping you both in my prayers.
Hugs