My son just turned 5 this July. He has behavior problems I have had him enrolled in about 3 different schools( Not this year - in about a 3 year time frame).

I have gotten a call from school twice this week asking me to get my son from school because his behavior is uncalled for yelling, kicking the desks spitting in the teachers faces,etc.. He recently saw a behavioral therapist and I don't think that helped much. 

He has a hard time sitting down and focusing on one subject for example Having him do a homework project which is just writing his name he doesn't seem to want to focus on this).  

His teacher has told me that if there doing a group activity he does not want to participate but if its one on one with the teacher he does a fantastic job and is eager to learn.. I'm worried because school will start in about 2 weeks and I am nervous on how he will respond to the change. 

My son is very smart and bright always on the go always wants to be outside doing something, he is very caring and loving.

His father and I are not together I feel like I am the only parent that provides discipline for my child, when he goes to his fathers house which is every other weekend he comes back with an attitude doesn't want to listen I feel that when he goes to his fathers he thinks its like Disney because he does what ever he wants. 

I have tried spanking but this does not seem to work as he tells me this doesn't hurt.

If I try and talk to him he shuts down doesn't want to talk or if he doesn't I'll threaten him saying he's going to bed when he gets home or not going to your fathers house this weekend (bc that's what he looks forward too) he will then say I am not talking to you anymore I am not talking to anyone.

I've had him tell me I am mean. 
I need some help with what I can do to change my behavior and also suggestions to help him?