his passport.

as long as we have a the receipt notice of our I-485 application, he is safe?

Yes. After he is laid off, he is in AOS.

Does he need to present other documents besides his AP when we enter the US?

His passport.
For safe, he can bring I-485 receipt with him (從 11/1/07 始 I-485 收據已不再需要)

For detail

閑話 AP (I-131) 和 EAD (I-765)
http://blog.wenxuecity.com/blogview.php?date=200706&postID=42162
再談 AP 和 EAD 對 F/H 身份之影響
http://blog.wenxuecity.com/blogview.php?date=200710&postID=20710