My 20 year old son is in his low mood & is not sharing the things exactly happens with him since about two months . I have consult the psychiatrist but he could not talk the dr. about his feelings . I am much worried about my son , only the medi. Is started but his behaviour is not like an engineering student. He is a bright student of third year but is not cope up with the situation . What should I do that make him happy.

Where are you located, and does he live at home while in school?
You say 'since 2 months.' You have no idea what started the depression?
Psychiatrists rarely do talk therapy anymore because insurance doesn't cover it. They just prescribe medication after a brief talk about what is going on. It's not surprising that your son didn't say anything. He needs a talk therapist.
Hopefully he has a situational depression related to some event, like unrequited love, or being involved in a tragedy. A broader depression might be realizing that he doesn't want to be an engineer, or that he's gay. (Just examples, could be anything!)
In any case, a good therapist should help a lot.
If he is depressed for no known reason, even after some talk therapy, then you take it from there (possible slow developing psychosis), but that is much more rare, so PLEASE don't anticipate or dwell on that. Try to remain hopeful. And know that medication rarely works all by itself. The therapy is vital.