sofardown: God bless you, you did the right thing for your son. I too had to ask my son to leave and after time had to ask him to stay away from me because it was just too hard to take a front row seat to his addiction.
People who are recovering from addiction will tell you how you did the right thing. I know their input is such a blessing for me.
I have to address your phrase about if your son did not have back pain then this decision for you would be cut and dried. May i suggest that is your guilt talking to you. My son does not have any medical reason to self-medicate, yet I feel the very same ambivalent feelings you do. And may i suggest that if your son did not have back pain your mom-brain would come up with some other reason for him possibly using (i failed somehow in raising him, I should have read to him more, I should have disciplined him more, I should have disciplined him less, I should have prayed for him more, i should have handled that one situation differently, i should have handled that other situation differently, he's just one of those people who takes a long time to grow up, i somehow failed in showing him that adulthood was something to look forward to, i should have put him in religious school....).
Trust me, stick to your guns on this one. It does get easier...