I'm sorry to learn of your son's situation. It sounds very challenging and sad. No one on will have better information on this than his current lawyer. How much incarceration will depend on many factors such as prior history, circumstances in aggravation with the instant offense, what the charges he pled to were, etc. You won't get a precise answer here.
You know your son best and how to handle the situation, but I wouldn't recommend opting out of Prop. 36. The fact that our government wants to punish people for their addictions is horrible, but Prop. 36 is a good step in the right direction. The treatment programs it requires could save your son's life and are a much better option than incarceration.
Another issue of opting out is your son being stuck with a felony conviction. A benefit of Prop 36 is that the conviction is dismissed when he completed the program. A sentence can be temporary but a felony conviction can have long lasting effects.