Dashmi - my boyfriend is also an ex-heroin addict, he had the same problem after he stopped. Heroin messes with your brain's serotonin levels, and when you stop doing it, you brain will take a while to go back to normal.
Maybe if he spoke to a doctor about it, he could get a prescription or something. My boyfriend didn't say anything to his doctor about it. After about three months he was pretty much back to normal.
Honestly, the best thing you can do is keep trying! And if it's too fast or he can't stay hard, he can focus his attention on you to keep you happy until he gets better. =)
So, I can say at least in my case that it did get better, it just takes some patience. That stuff really messes up your body and he just needs some time to get back to normal. Don't get down on yourself thinking it must have something to do with you!