What does "stand by him" mean? I understand that he asked you to take him to detox, but stand by him is less clear.
Again, I suspect that by keeping thoughts about the relationship vague, you do not have to really look at what is going on. Have there been any cancelled plans? Have you had to deal with him when he is high? Do you end up paying for meals? Can you talk honestly to him about the times when you feel hurt or angry? In what ways does he stand by you?
It is hard to imagine that someone using heroin has not disappointed you in some way and at some time. And your support has not improved his situation. In fact, from your description it sounds as if he has gotten worse since your relationship has gotten serious.
As long as the relationship is comfortable for him, all the love and support in the world will not motivate change. Why should he change if you are willing to stand by him all the way even if he decides to go down the drain?
Setting some limits for yourself can only help. How far will you go? How far is too far? Decide now before the time comes because if he does continue on this path you will always find some excuse to stick with him even when the situation gets intolerable.
You can still hope it does not come to that. But you need to plan for the worst as well as the best.