There's an old saying that men will eventually treat their wives the same way they treated their mothers. There is nothing wrong with a man who loves and treats his mother well; in fact, it might even bode well for a significant other. There's a difference, however, between a man who loves his mother and one who is a "mama's boy." In a relationship with a mama's boy, you'll find that you always take a backseat to the wants and needs of his mother. Luckily, if you learn the warning signs that he's a mama's boy, you can dodge that bullet before you get into anything serious with him.
He won't leave the nest. Adults may live with their parents for a variety of reasons nowadays, and some of them are worth the benefit of the doubt. In some cases, however, it's a red flag. Does he pay rent or do housework? Does he express a desire to get his own place eventually? If the answers are all "no," you might be dealing with a mama's boy.
He turns into a thumb-sucking toddler around her. When a mama's boy is around his mother, he instantly transforms into a different person. The same guy who was guzzling beers with his buddies earlier is suddenly yelling to his mommy from the couch to bring him some cookies and his favorite drink. This is especially true when he is sick -- in which case, a mama's boy might very well stay home from work and call his mother to come take care of him all day.
He lets her interfere with his relationships. A decent man might care about what his mother thinks, but for a mama's boy, it's the single deciding factor. Does he call her up every time you have a fight and share all your personal business? In arguments, does he bring up her opinion instead of his own? Does he refuse to date anyone she doesn't approve of? If so, he's probably a mama's boy.