mama's boy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A boy or man who is considered to be overly close to his mother and often timid or overprotected.

from the GNU version of the Collaborative International Dictionary of English.

• noun Same as mamma's boy.
• noun A boy excessively attached to his mother; a boy lacking the usual masculine interests; also spelled mamma's boy.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, derogatory A male person, especially a young man or boy, who is overly attached to or influenced by his mother; a sissy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a boy excessively attached to his mother; lacking normal masculine interests