My boyfriend used to be a drug addict. What s the chances of him relapsing?

So my boyfriend used to be a hard drug addict in which he spends years on gateway drugs and that further led to an opioid addiction. First with prescription drugs and then that led to heroin. After two years on heroin, he decided to quit and has been clean for 10 years. He reassure s me that he d never touch those substances again by telling me that while recovering he lived with an addict and still never touched the stuff. Also, all of his friends who used are now clean too. I know he would never want to hurt me, he used to smoke marijuana and chew tobacco when I first met him but stopped when I addressed that I didn t want to be involved with someone that did that stuff. So, I know he s capable of never touching hard substances again but I never know. I love the man to death and I know he wants to spend the future with me but I m scared to death that he may relapse on me. He knows how scared I am and I hate to address him on the mistakes he made in his past, so I very rarely bring it up. But, what s the likelihood of a relapse because he s pretty confident it will never happen but my best friends dad was 20 years clean and relapsed and has been struggling ever since.