My own opinion is that two days a week is too often to call him right now. He hasn't spoken with you for several months up until today, right? I just think twice a week might scare him a little, at least for now.
I'd start off with once a week, and if that seems to be too much for him to handle, cut it back to once every other week. One thing you might want to think about is sending him occasional greeting cards through the mail. I know my son enjoys receiving a "thinking of you" card from me every now and then. (He usually calls me right up after I send him a card.)
You're probably going to have communication problems in your relationship with your son until he stops using drugs and/or alcohol altogether. Drug addiction can complicate a reunion relationship very easily and very quickly. Do you know if he's tried going to a Twelve-Step program? They really do work, one day at a time.
Hang in there. It's still real early in your reunion time-wise. I know that you're probably worried sick about him. I'm just afraid, though, that you'll scare him away if you contact him too often, at least right now.
