Informal word for someone who likes to keep to themselves

Solution 1:

Loner

Merriam Webster's definition of loner:

loner, noun | lon·er | \ˈlō-nər\ : one that avoids others; especially : individualist

Personally, I have found that anti-social, while sounding technical, is used far more in the sense of being a loner.

Why doesn't my son want to play ball with the other kids? He's so anti-social. What will I ever do?

Solution 2:

Consider,

stick in the mud

(idiomatic, pejorative) A person unwilling to participate in activities; a curmudgeon or party pooper. Have a little fun sometimes and don't be such a stick-in-the-mud. Wiktionary

homebody

A person who prefers pleasures and activities that center around the home; stay-at-home. Random House

Solution 3:

Many learned people use 'anti-social' to refer to those who keep to themselves. But I think in many parts of the world the word is used to refer to criminals. There are many alternative words. Examples: Don is a private person. He does not attend parties. The tribal people are usually unsociable.
The president is media shy. It is difficult to get him talk. Kala is an introvert. She lives a solitary life.