Want one of my personal fat loss secrets?

So Son o'mine has been using my 5lb weights to work out for some time now. I just bought him a 10lb weight this weekend, now I'm thinking maybe it was a mistake, he's 13 is that too young to be using a weight that heavy, or is it ok? I've heard so many different stories I don't know what to believe. How do I know if he's using it properly? The one I bought him looks exactly like the one here.