I agree that you should give it some time and give him space. At the point at which you decide to re-engage (if you do), you have a right to ask some hard questions. Such as, how will you stay clean this time? What triggered your relapse?
The other thing I would want to know is how he came to be an addict. If he came to heroin from a medical issue, such as opiates prescribed after an accident, and not from an emotional or psychological issue, then I would think that the prospects are better for eventual recovery.
I would also be very cautious about getting involved with an addict. At 26, you have some time to explore other relationships.