What he first will think?

[Ay__en__]

A girl who started suddenly ingoring a boy and he knows she likes him
(they never talk, they just make eye contact)

[Noodle_fan]

he would assume she is shy
however it is also depend on how he knows she like him , if she is shy there is no way she would give obvious sign about it

[Ay__en__]

and if she isn't shy

[Noodle_fan]

may be she is upset with him over something

[Ay__en__]

that what he will think in?

[Noodle_fan]

hard to tell without knowing his personality
so i can just randomly guess
what he think about it? may be he wouldn't care , who know?

[Anonymous]

I might think
Oh she’s upset at me or wait is she trying to get my attention?

[serious]

Well the guy will thinks she is playing mind games and so will back off and then may end contact with her. Simple

[spartan55]

How does he know she likes him if they never talk?

[Ay__en__]

His friends tell him that, and she acts she likes him and he noticed that, they make eye contact
(Before she ingored him, he was acting like he likes her, but like acting, cause he doesn't care at first and she who try hard to get (

[spartan55]

He is likely so confused at this point.

[Ay__en__]

What he will think then?

[spartan55]

He might be tired of all the games and not respond.