Hi Kathy and welcome. My son is an addict and has been through rehab several times, and I know that the first week or two can be the hardest for them probably because they are still detoxing and also they are facing the world clean for the firt time in a long time, and feeling emotions that have been numbed for far too long.
Leaving him there is the right thing to do. As you said, he can leave if he chooses, but by not helping him he may decide to stay, and even if he leaves it would be his choice alone.
If you and your husband have not been to a Nar-Anon or Al-Anon meeting, make a date and go. You wil meet many people there who have gone through this and they will help you find peace again and learn how to step out of his chaos. There are a lot of moms here, on the Nar-Anon forum, so please feel welcome to come down there and say hello. Take a read around and just make yourself comfortable, and know that you have come to a wonderful place of hope and support.
Hugs
Ann