What were your choices? Have him stay and let druggies ultimately come to your home for their high? Have him stay and worry that he doesn't or can't pay for the drugs so the ones above come to your home to harm whomever is there to get their money? Have him stay and have the police come and arrest you for drug possession in your home? Those are all very real scenarios. You truly had no choice. Your son did and his choice was to put himself and those who love him at risk.
I'm sorry this was what you had to do. Unless and until he is ready to truly change, quit using drugs (few dealers don't use and those few are usually at the very top of the drug chain), gets himself into and completes rehab, he isn't your son -- he's a person addicted to drugs. Don't entirely give up on him. He might turn himself around. It has been known to happen, there are even a few shining examples on this board.
For now, many, many hugs. Come and talk to us when you're lonely, grieving or doubting. We'll be here for you. We understand your pain and your anger and your sorrow.
((((((((HUGS)))))))))