This is where you stand tall and with all the grace and strength you have you SHOW your boy the best of you. Show him the high tensile steel you have in your veins. I know it's there.
He shouldn't replace the lunch box. If he's like most kids he may be a bit careless. Sometimes as parents we overreact. I'm hoping that is what his Dad did. However, it's a lunch box! Protect yourself and your boy. Talk to him and let him know his value does not come from other people. Show him this is true by your own actions. There is a book by Stephen Stosny (?) about walking on eggshells, I don't remember the title off hand. I'll look, but you might take a trip to a local library...
Have you talked to someone about your credit? Perhaps there is something you can do to start fixing it.
