Desperate, welcome to SR! I do understand seeking any opportunities available to help your son. I have been there with two daughters myself.
Family members are affected by alcoholism/addiction, and the anger that you are starting to feel is natural.
I would suggest looking for Alanon meetings in your area to find face-to-face support among people who understand and have been where you are at.
A four month program is a good start, but just remember that your son will get out of treatment what he is willing to put into it, and your husband and MIL are correct in stating that it isn't a cure.
The best thing that you can do for your son at this time while he is in treatment is to get support for yourself and start healing from the effects that his addiction has had on you.
I hope you continue to post, and know that you are among friends.
:ghug
PS. I also suggest the book "Codependent No More" by Melody Beattie.