If an adult boy shows these following signs then is there any possibilities that he has a girlfriend (A committed girlfriend.)

1) Ignoring mom ( Not rude).
2) Suddenly starts being serious at home
3) Suddenly starts being polite with everyone
4) Now on social site (but not sharing the id name with family)
5) blushing on romantic funny scenes ( scenes like a man flirting with a woman )
6) Doesn't smile at home (like being serious )
7) suddenly starts listening really Good Romantic songs.
8) Being particular about the dressing and figure.
9) blushes and gives a big smile when asked questions like (did you go to see this movie ?)

Oh my goodness! Sure, your list is indicative of just about anything involving romance, but it could be a crush only, a one sided crush, all the way to 'committed girlfriend.'

Why not just jokingly ask him? When he plays a romantic song, ask him to dance around the room, and say 'Is she a good dancer?' When he blushes, ask 'Is there a girl who makes you blush too?' Mothers have their ways to find out, without being pushy and driving sons away.

OOPS, I see that you are a teenage girl, so this must be your brother. Just ask him, but if he doesn't tell you, leave the poor man alone.

Yes, it can be there, or a sign of a 100 other things,
Boys go though stages.

Um... to start with... If he's an adult... he is a man, not a boy.

If he is adult like you are saying, it is normal for the man. Your concern about he has a girlfriend ? Well, he would have many crushes as he is adult. Be relaxed if a man finds a woman that too a committed one, he would not afraid to share it with anyone specially the family members ! As joypulv said that you are a teenager, you should not mind about his crushes. As long as he is the responsible one.