Just because a dog is of a certain breed doesn't guarantee the temperment of that dog. Each dog is unique in how it will react with other dogs/humans/animals. While there may be some breeds that are more prone to aggressiveness, every dog has the capacity to be aggressive to some degree or another.
I'd say your best bet is to take your current puppy with you to meet any potential puppies for your brother. That way the two can meet, play, whatever, and you can see how they interact. If you're getting the new puppy from a breeder, ask the breeder lots of questions about the puppy's behavior, attitude, socialization, etc.. That will give you a better picture of what you'd be getting yourself into.
I can tell you from personal experience that my puppy knows who it likes and doesn't like. He's a Welsh Corgi, so he's short in stature, and he doesn't like taller dogs - but he'll romp and play with dogs his size. Around taller dogs he just cowers and hides, until they meet nicely, and then it's all good between them.
And make sure your brother knows what he's getting himself into. A puppy is a 24/7 commitment, and a BIG commitment for a 12 year old. I had no idea what I was getting myself into when I got my puppy, I had no idea they'd be this demanding, and I'm 25 years old and home all the time. But I've learned from my mistakes, and it's all gravy now.
Lastly, there's no need to get upset and be rude with anyone on this board for not answering your questions right away. You asked a very open-ended, broad question, that will take some time to answer. Not everyone has the time to sit down and give you a detailed response. If you're not getting the responses you want, or responses fast enough, try narrowing down your question, or doing your own research on the web while you wait for a response to your question. Besides, you've got several months before you even get the new puppy, so you've got plenty of time to research and learn before then. And encourage your brother to do the same, so he has an idea of what he's getting into, and what he'll be looking for in a puppy.
-Tim