Help!

I have a step son who is almost 13years old and he is with us every tuesday/thursday 4-8pm and every other weekend. just the other day i was missing money from my purse. and it has been happening ever so often. and I started thinking its always the day after he was here. but never feeling 100% sure i just can't remember where i spent the $20 . this week is different and i am 100% sure i didn't spend the money and it can't be a ghost taking the money. Monday(labor day ) i had a yard sale. I had $10 in quarter and put it in my wallet which made it very heavy. I also had bills in my wallet. on tuesday i went out and didn't touch the quaters. I would remember using $10 of quarters. on tuesday afternoon he was here and then last night(wednesday) i went to get quarters for my two little ones for school, they have been asking me for milk money since school started, so i finally have lots of quarters. i look in my bag and all the quarters are gone. my husband said he didn't take them. and my little kids are too young to take them. (6 & 5 years old). so tonight when he was here, his father/myhusband asking him if he took the money. he very quickly says "NO" and then i told him so basically you are tellin me a ghost took my money. so now my question is for all you mothers who have children and espeically mothers who have a stepson how would you deal with this? i can't just say ok, and believe him. the money had to disappear some how and its not my husband or little kids. and i know that i didn't spend them or put them somewhere else. it has to be him and the other 4 times it is most likely him too. he has moments when he is here that he is more than able to sneak over and take the money. especially bills its quiet. and i am so angry about this. he is truely lying to us about this. he has lied to us once before but it was something silly. and his mother lies to us all the time and he knows it and laughs about that she thinks we don't know. but we do. so he is learning from her its ok as long as you don't get caught. how can we handle this. its so hard to take stuff away since he isn't here much. please help and soon so i can't deal with this. this weekend he will be here.