UDK is the exact same as the full UE3 engine.
I also have unity, but it requires scripting, which, depending on the age of your brother, that may be a problem. The free version of Unity is a cut down version as well. One of the problems being that it doesn't have shadows :/
If your brother has no programming experience, get him to try game maker. https://www.yoyogames.com/gamemaker/
Imagination is more important than knowledge.