Why are you being the money warden? If he's going to use... he probably doesn't need money. My husband didn't. They pick up for friends and make extra for buying in bulk.
For me, using wasn't the problem. It was the behaviors that stemmed from the using. Lack of money (not being able to pay his bills), stealing, not being able to get out of bed....
I'm not saying leave your kids with a drug-addict ... but if he's acting responsibly...
There is always the risk of OD'ing, which I really really get... but with him living in the house that can happen at any time.
I would really stop stressing so much about IF he's using, and focus more on YOUR behavior and HIS behavior.
There are tell-tale signs. Driving yourself crazy over if he slipped will just make you nuts. I know, from experience. Let it go. What will be will be.