Please Help With Drug Abuse…?

my boyfriend has been doing cocaine for the last 3 years, hes been in rehab more than 3 times already but is not really working. He stays sober for maybe 1 or 2 months, but then there he goes again.. He tells me that he wants to change but its just to hard because he gets like cravings , is there a way that he can get help or something cus hes been getting worst everytime… i dont know how to help him anymore and im really worried about him. please help!