So sorry you’re going through this Mother-1, welcome to the board. We are all in the same situation with “helping” our adult children. We must think of ourselves more and let our children go, to figure their own life and the messes they get in for themselves. You absolutely are NOT being too harsh, you did the very best thing in this situation….letting him know once and for all you will not tolerate drugs in your house and the disrespect he gives you.
My own daughter is in jail right now, again. She is 32 years old, we tried for 10 years of helping her. She took our money that we paid her bills with, gave her a place to live in our home - disrespecting all rules we put into place bailing her out of jail each time, drove her around to all court appointments, 5am surprise drug testing, to a job until she had gotten her license back and a car. This process took about 3 years of our life away each time and she is back where we started. In jail, license is suspended yet again, car was repossessed. She has nothing and we will not be helping this time. Enough is enough!
Another good book is by Allison Bottke Setting Boundaries With Your Adult Children. She writes from personal experience with her son, being enabling and co-dependent on our children. Her son used and sold drugs as well.
Trust me! I want so badly to come in and rescue my daughter as you your son. In fact she just called from jail and we refused the call. Would love to talk to her as I have not seen/heard from her in over a year. I won’t let my weak heart give in this time and the games begin again.
Stay strong, you really are doing the right thing.
Big Hugs! 
