GandalfTheWise

The main point is this. He has had no interest in Christianity in 6 years and sees himself as Muslim.  At "best", putting an ultimatum to him will likely result in a superficial conversion to make you happy in order to get married.  This would likely make him some combination of apostate or pariah among muslims including his family which they would be angry at you for or you'd find he didn't really mean it.  The reality is that people change in marriage.   Those deep burning feelings of passion slowly disappear over time and need to be replaced by common purpose, comfort, compatibility, and a sense of security and familiarity with each other.  It frankly sounds to me like you are still in the passion/infatuation stage.  What happens if he has a superficial conversion to please you now and ten years from now he blames you for loss of family relations?   I've heard many testimonies over the years and have heard many more "forced" romantic conversions that were not sincere and did not last than I have such conversions being long lasting.  I've heard a few, but they are rare.  The majority of testimonies I've heard from Christian women later in life who were in your situation either regretted staying in such a relationship for years or decades or expressed temporary pain at breaking it off but were happy they did so.  I can only recall a few where the husband did indeed become a practicing Christian.
If you decide to break this off, make the decision to move out and set the plans to do so.  Have family or friends ready to help you move out and into a place where you will have some support from friends or family.  As to what to say, I think the most important thing is honesty and integrity.  Your relationship with Christ is more important to you than a romantic relationship with anyone so you must end this relationship.   Don't turn it into an ultimatum to convert.  Just present it as a fact and leave.  In the long run, if he becomes a Christian, it cannot be for the motive of being with you.  It must be because he comes to follow Christ on his own and determines that Jesus Christ is the focus of his life.  He must be willing to face the inevitable pressure and repercussions from family and other Muslims because he loves Jesus Christ and not for a romantic relationship.