Hi mate, I’m at work so can’t reply much but I saw you say you feel guilty for your son who just wants his daddy.
If this has been going on for 6 years already then the best chance of your son getting his daddy back is to distance yourself from your partner. Make sure that he knows that he needs to get help and control his addiction so he can be the dad his son deserves and hope that the realisation of you leaving and it affecting his relationship with his son helps him get clean.
If you do nothing all that will happen is your son will have a dad who’s doing gear and a mum who is worn down and tired from trying to hold the family together.
Be strong and always do what you know is right by your son.
Take care
James x