So sorry you are here. If you found a syringe, he's most likely using again. Read, read, read here. Use protection if you are sexually active, if you choose to be sexually active after what you've found.
Heroin is I believe the if not one of the hardest drugs to get off of. Last I knew statistics were around 11% success rate. My xah's DOC was heroin, but what started it was oxy. There will be more that comes along with wiser advice. Just know you're not alone, but from here on out trust your gut. NOT him and protect yourself. Physically AND financially.