8yr old son caught with stolen money...

So today I woke up to a message from my boys mum that she'd found £15 in his room last night, taken from her partners wallet.

To our knowledge this is the first and only time this has happened, she tells me they've both had a talk with him about how it's wrong and what not but he seems entirely uninterested, he's lost access to his tech and things for a week but again is completely uninterested and is acting as if he doesn't care, doesn't want to apologise etc.

We're really not sure why he's done it either, usually if he asks for money from her partner he gets it, he gets weekly spends from me that gets put onto a card he can use, which he knows about and knows how much is on so it's not like he doesn't have his own money to spend.

We're not really sure what else we can do to help him understand that what he's done is wrong, anyone got any advice? I've even suggested she takes him down to the local police station for a chat at the front desk but I'm fully expecting her to ignore that suggestion or tell me it's ridiculous not doing that anymore.

Edit, just adding thank you to everyone that has taken the time to respond, I'll get to the comments tomorrow and respond, life being life threw me another curveball that needs my immediate attention.