Share all the wonderful things about your son’s father with him. He will also know the truth about his father’s addiction. It is an illness and the only one to want to cure it and get clean is the user.
Our husbands loved drugs more than they loved us - does not mean they didn’t love us too.
My former husband is still alive, still doing whatever drugs he wants, he had and has no intention of quitting, he loves drugs. Not long after we got married he told me he reserved the “right to do heroin if he ever needed - or wanted it.”
I am sorry you and your son have your entire lives ahead with only his memory. Find peace.