Why was HEC so mean?

I am pretty sure that by "HEC" the question is asking about Herman E. Calloway. Additionally, I am sure that the question is implying that Calloway is being mean to Bud.
Calloway is mean to Bud because Calloway knows that Bud is lying about being his son. Bud believes that Calloway is his father, so Bud goes looking for Calloway in order to live with a family member again. Calloway knows that he only fathered a daughter, so he knows that Bud isn't his son. This causes Calloway to think that Bud is just some runaway looking to get some easy room and board.  

You see, kid, you ain't the only one trying to get on Mr. C's good side.

The story takes place during the Great Depression, and Calloway is one of the fortunate individuals that can maintain some kind of employment. He is financially successful at a time when people are struggling to put food on their plates. Bud is probably not the first person trying to con Calloway out of some money. Calloway sees no reason to be nice to a kid that he knows is not his son.  

He was so doggone mean and hard to get along with it just didn't seem like it was true that he could be anyone's daddy. The way he was so worried about me stealing stuff from him before he even knew if I was honest or not made me wonder if someone who was so suspicious could ever be kin to me.

Of course all of that changes when Calloway realizes that Bud is his grandson. Calloway feels horrible for how he treated Bud all while being happy that he has a grandson from his daughter.  

You can't help but feel sorry for him, even if he's been mean to you from the minute he first had eyes on you, even if he's crying 'cause he found out the two of you were kin.