You did the right thing. By detaching with love we allow them to learn how to take care of themselves. Sometimes this means also learning that there are consequences to their actions, sometimes it allows them to decide to reach out for reecovery.
Regardless of what they decide, it's their journey and not ours to direct.
If he is unkind and disrespectful, you don't have to participate. I have a boundary that as soon as a conversations becomes heated or disrespectful, it is over...right then and there. I leave or hang up and don't have to slam the door or phone, I just need to detach. I respect myself enough today to not allow verbal abuse. That took me a while to learn.
I don't know if you've been to any meetings, but if you haven't maybe give them a try. Meetings literally saved my life and many here have found great relief in having live support available to them.
Keeping your son in my prayers, and you also. It's just not easy being the mom of an addict.
Hugs