He chose heroin over us


My partner has just walked out on me and my son and has chosen to live with a woman who also uses heroin.
My partner is a heroin addict and I have been with him for 6 years. We have a 2 year old son together. 
I have tried all ways to help him but nothing has worked. I just wanted him to be a good dad to our son but he chose heroin over us. 
I am going through a mix of emotions from being heart broken to being angry. 
He is a clever man and could have been successful in anything he wanted to do but this drug has taken over his life - that's if you can call it a life he is living. 
Why would someone who adores our son walk away and not even get in touch to see how he is doing? I hate heroin and I am just beside myself. Has anyone on your blog got any advice for me?