Hi, I'd like to help you with your question.
It sounds like your son needs drug and alcohol treatment, possibly inpatient. Have you tried confronting him with what you know? If you have and he denies having a problem, then he probably will not be willing to get help on his own. If he recognizes having a problem, see if you can talk with him with your husband and get him into treatment. If he is a veteran, there should be treatment available at the VA.
I'm not sure how he is gaining access to your checking accounts, but if he knows your account numbers, I would change accounts immediately. The same with any other accounts you have that he may have access to. You may also want to file for full custody of your granddaughter. If your son is stealing to maintain his habit, then he may not be in any condition to care for his daughter.
How you handle it from here depends on whether or not you are willing to expose his drug abuse. One option is to call the police. This is a drastic measure, but it would ensure that he gets help because the legal system will be involved. Another choice is to contact his doctor who prescribed the meds and let him/her know that your son is abusing the meds, needs switched to another med and needs drug abuse treatment as well. You can also try an intervention. This is where your family gets together and all of you confront him about his use and his stealing. This is best done with a professional involved to help guide the process and ensure everyone doesn't overreact or become too upset. A good website to help you with an intervention is http://www.drugandalcoholrehab.net/Intervention.html.
Some books that may help you include My Daughter's Addiction: A Thief in the Family - Hardwired for Heroin by Marie Minnich and Motivating Substance Abusers to Enter Treatment: Working with Family Members by Jane Ellen Smith and Robert J. Meyers. You can find these on Amazon.com or your local library may have them.
I hope this has helped you,