Tell him he's precious, talk to him about how he feels about his favourite belonging - then tell him he's a million times more precious to you than that.
Tell him like he has to care for his stuff and make sure it's safe you have to do that for him too. Tell him Mummies have to know people really well and trust them for years before they let their bestest thing in the world stay there. Perhaps say that he needs more time to get to know Caleb as a friend first or it might spoil the friendship.
7 yr olds can't keep secrets - what you tell him could well be said to Caleb and maybe his parents.