I think you've been remarkably open to people's (very good) advice on this thread - serious kudos to you, toffee.
Just wanted to add that I went through a stealing phase when I was about 10. Nothing was wrong - lovely parents, happy life. I just wanted things very, very badly and didn't have enough self-control to not do it. I almost went into another mental state when I did it, it wasn't really 'me', which is probably the state most children get into and why they can swear blind they didn't do it even in the face of all evidence.
And I stole. I properly stole. Food. Coins from my dad's change bowl. Bath pearls from The Body Shop. A packet of crisps from a friend at school (of of all the things I took that could have had serious longterm effects, no-one likes a stealer). And (this almost sounds like a joke) a silver snuff box from a vicar.
When my mum confronted me about it I denied twice then burst into tears, I was so horrified that I'd taken it but as much that I'd been caught. Mum returned the snuff box, made me write a letter of apology and rather than screaming at me, hugged me and asked if anything was wrong. That kindness stopped me in my tracks and I never stole again.
I look back on it now and I can hardly believe it. I was a very honest, honourable child, quite bookish. I was on the cusp of adolescence, about to go to a new school blah blah blah but basically I did it because I could.
But I wanted to be clear: my parents did not cause my stealing. they were excellent parents, and their gentle reaction shamed me and allowed me to move on.
I am a very honest and honourable adult and I certainly don't steal 
Don't blame this on yourself, keeping reminding him gently that stealing isn't nice, and just as you say, think 'will I even remember in 2 years'.