Small children don't have full control of their strength so I wouldn't recommend a small bird like a budgie unless you know your brother is a naturally gentle fellow. I sometimes scare myself when I touch one of my birds a little rougher than usual. They're just too fragile for young children.
Whoops! Just noticed your brother's age. In this case, I think a 12-year-old can take care of a small bird very well.

A friend of mine has a rat and I think it's absolutely adorable. Since your brother has experience, I would also recommend either another bunny or rat. If size is an issue or you want to try something different, you could go with a different breed of bunny. I'm sure your local shelter has tons of buns (I know mine does!).