Thank you for your help.
1. He's a book.
2. He has a book.
(Does #1 mean #2? Can't we contract "He has"?)
3. He's my uncle.
4. He is my uncle.
(Here, the contracted form is correct,isn't it?)
#1 is incorrect for the reason I gave here:
http://www.jiskha.com/display.cgi?id=1488799319#1488799319.1488800406
#s 3 and 4 are fine because the main verb is a form of the verb to be.