Is he running 1.6 or 2.1?
If it is the former have him upgrade to Android 2.1.
And if he's already using 2.1 backup all his data and do a phone hard reset. Also, tell him it's a bad idea to use a task killer application as Android is smart enough to manage memory on it's own. In fact, having a Task Killer running in the background and having it auto-killing applications is actually a waste of CPU cycles.
Make sure you and your brother read this article:
Android Task Killers Explained: What They Do and Why You Shouldn't Use Them
It explains in detail why task killers are BAD for android.