Okay let me emphasize I am not a professional but I see a few things going on here.

1 - For some they desire to be with someone like this because ultimately they hope that they are the "answer" or the reason the person ends their addiction. This may not be you but it is more common than you think.

It is similar to the "groupie" who wants to hook up with the celeb so that she is the one out of everyone he could have had that he chose to be with.

2 - You have a delusional perception of your relationship. On some level he believes he cares for you but you are likely the ace in the hole or the ground in his electric life. I have a music friend who just married a guy like this and she found it that he had sex with dozens of women since their nuptials! Prostitutes, swingers, and anyone he could pick up. Oh and get this she was "spying" on him too! But that turned up nothing. It was the guys sister who forced him to tell the truth.

My friend long suspected but she felt some warped loyalty based on nothing stable. That is exactly what you are doing!

If the guy did really care for you that much he would have long addressed his issues and sought counseling and treatment the second he realized he had true feelings for you and that he wanted to have a monogamous relationship with you. He did not just become this way yesterday.

3 - There are also some who get in a situation like this because of their own self esteem issues. Look no one deserves this or needs anything remotely like this. Unless you have some hidden "swinger" in you. Which is an entirely different issue.

Note: He is a user of human beings and their emotions

In conclusion it is clear:

If you care about him and/or yourself even a little bit than you know what you need to do. The supportive thing for him is too leave him and tell him to get back to you if and when he has transformed himself into a self respecting man instead of a greedy lust driven self gratifier who has little respect for the feelings of those who he is committed too. Yes he likely has a sickness, so let him get help.

I mean if you learn someone is an alcoholic before you are all in then is your chance to lay down the law and make the decision best for them and YOU!

There are certain unwritten codes in life: Here's a couple.

IF you need a gun to feel safe at a restaurant, that is not the kind of place worth eating at

IF you have to spy on someone to feel a sense of control or trust than it is over before it started and you are in for nothing but trouble. No relationship could be worth the effects of that.