Why did Ricky's dad kick him out?

Ricky's father, Frank Fitts (played by Chris Cooper) is hyper-controlling and wants to maintain a strict control over both his wife and Ricky - and does so with an ever-present threat of physical violence.

It also becomes apparent that Frank has some repressed homosexual feelings. This is evident from the later scene where he unexpectedly gives into these feelings with Lester (Kevin Spacey), and Lester gently and sympathetically turns him down.

To my interpretation, Frank is repressing his homosexual feelings and hates these things in himself, as they are far removed from what he believes are right and proper.

Ricky on the other hand is more of a free-spirit, and although he does not openly challenge his father until the end of the movie, he clearly wants to live his life in a different way, a way that his father does not understand.

Frank sees Ricky become friends with Lester, and given his own repressed sexuality, is suspicious of the relationship. Presumably (although it is not explicitly shown) he is likely to know that Lester gave up a good job, and sees him buy a sports car and start using weights in his garage, and spend time with his son often furtively as Ricky is selling Lester pot. He is attracted to Lester but repulsed by the lifestyle that he thinks goes along with that. When he challenges Ricky to admit that he is gay - he is not surprised that Ricky says yes because of his own sexuality, and a misunderstanding of his son's alternative lifestyle. The idea repulses him, doubly so because of his own struggles with his sexuality, and he throws Ricky out of the house as he cannot deal with the emotions he is feeling.

This dual fascination and fear/loathing of his own sexuality leads onto even more extreme consequences when he cannot deal with the realization that Lester and presumably Ricky are in fact not gay.