First of all, your willingness to listen to him with an open heart - without judgement - is what he needs most of all. It sounds as though you are trying to do just that.
What is in the past, is just that. Mistakes are made for many reasons.
Most people don't know a fraction of the secrets a mate keeps... because too many people know that a partner may leave them after they expose those details.

Every day is brand new. If he is able to stay healthy and clean today, that is all that matters. And the fact that he is willing to be so honest about his past shows very strong character.