My brother's eighteen year old friend recently asked me if I wanted to date him in the beginning of August.Then once school started, he began ignoring me but keeps staring at me and comes over to my house and talks to me only when my brothers around. Is he shy or just moving on?

If he's ignoring you other wise, from personal experience I'd say he's shy.

When he asked you out, what did you say? That would help me answer it a little better. If it still seems like he's interested in you though, I'd guess it's that he is shy.

How old are you?
Assuming you are younger and he's 18, you are a minor and he's an adult. This means serious legal problems can arise, and he can be charged with various crimes regardless of how consenting you are. He may have decided this on his own or been warned.
Is he out of school?
Different grades or being out of school have a lot of effect on relationships at your age, even just in how friends view it and pressure from them or adults, so that might be part of it too.

I'm sorry but I think he has moved on hopefully you aren't in love with this guy he seems pretty... fishy...