My oldest son (grown now) went through a phase where he would pocket cash that wasnt his,i come home from work,go straight to my office & empty my pockets on my desk,i leave money on my desk all day every day & if anybody needs money all they need to do is ask & i give it to them.
My boy was spending way too much money on his girlfriend & his normal cash flow wasnt enough for him to be the cool cash spending guy he wanted to be, so he started snatching some cash from me.
I let it continue for a while,at first he was taking like $20 to $40 bucks a week,i kept watching him,when he graduated to taking $50 bills i confronted him,all i told him was that i was dissapointed as a mother f-er in him & explained how bad it made me feel that he would take money from me when i allready spoiled the shit out of everybody in the house,i made a point to tell him that up to that point all i had was reason to be gratefull & proud for what great children we were blessed with,after that i had reason to be ashamed of one of my son's,i even faked a tear or two
.
That was all it took,i never had one red cent come up missing ever since,with my children the worst punishment of all was mine & my wifes dissapointment in them & their actions,we never had to be super strict parents nor did we have to seriously punish them or ever put our hands on them.
Try explaining to him how much he has let you down & how sad his actions have made you & your wife.