It is very possible that he was a weekend warrier all along, sustaining the fantasy that he could control the beast until it once again, controlled him. Addiction refuses to compete. Addiction demands your husband to protect and sustain it at all and any costs. You don't know the truth. He probably does not know the truth.
How easily your child could have found what you did and injested it. Does your life plan include the potential for the ER pumping your baby's stomach, brain damage or worse?
Rehab does not cure addiction. At the very best, a rehab can teach the tools of recovery. It's up to the addict to use those tools or not. It's a fact that most former rehab guests relapse. Your husband did and did so despite pending charges, a business and a family.
Only thing certain is that he was not using at your or the baby. Addiction is not personal. It just feels that way to the people who get in its way.
You did not cause this. You cannot control this. You cannot cure this.
You can however protect your child, whatever it takes.