How to handle a stepson who steals!?

I’ve got a huge dilemma. Today I just found out my stepson has used my atm card and took a total of $140 from me within the past two days (he put the card back in my purse). What prompted me to check my bank account is because when I got in my car this morning, I noticed my car was on empty when I just put almost a ½ gallon in on Monday. I only work 5mi from home and don’t go anywhere else aside of that during the week. So I looked and saw an ATM withdrawl for $100 yesterday and $40 the day before. I called my bank and they said it happened before midnight on BOTH occasions. My husband and I are sleeping during that time. A few things that didn’t make sense before are now coming together due to this. I called my husband and was totally upset, he told me to not worry he’d handle this when he got home from work today. I told him I didn’t want to believe this because his son is so well mannered, respectful, is in college and does well all the way around. It made no sense…OH and HE ALREADY HAS A JOB! Granted it’s only pt but still WHY STEAL FROM ME!??? I told my husband if he even thinks he’s going to deny it, I will go to the bank this happened at BOTH TIMES and ask to see the security cameras. I’m going to be a part of this conversation once we get home, but I’m so speechless! I’m angry, hurt, and I don’t know how to even express that to his son, but not come off as a crazy woman! I want to just get my point across that he’s 18 years old and we don’t need to put up with this!