Just tell him the truth and let him know what the future my be like if you were to force yourself to be with him. Sometimes the spark may not be there anymore, because you both got comfortable. Maybe talking to him and telling him why you feel this way will change for the better to. However, if you know that you no longer love this person and is not willing to compromise anymore, then it's better to just let him go no matter how hard he tries to hold on. You don't want to do this to him 10 years later when he has already committed so much.