Help...7yr olds behavior problems.

My son is 7yrs old and in 2nd grade this year. When he was younger I never had any problems with him. He was fine in Kindergarten and then we moved half way through the year. When he started his new school he started having behavior problems like not listening in class goofing off and playing too rough or not socializing. The school said it wasn't anything to really worry about because it was probably the move that triggered it. Well 1st grade came around and it didn't stop. He never gets in trouble at home! He is the best kid when he is home, a great big brother, so helpful. 1st grade he was getting in trouble for the same things. Not doing his work, not listening when told, playing too rough, etc. I was in the office at least once a month. They took away his recess several times he was grounded and talked to at home, and I don't let him watch violent shows. I noticed 1st grade he stopped talking to me as much as he used to and started being shy around people. He doesn't like to interact with some of the other kids. This summer was good he hardly ever got in trouble at home he made a couple of friends that he played with a lot. Then 2nd grade came and started off good. He is now getting in trouble again. It started as before, not paying attention, etc. Now its getting worse. His teacher says he knows how to do the work he just doesn't. He doesn't concentrate on it he would rather goof off and draw on his paper or talk to others. Then this past 2 weeks he's been sent to the office twice. The first time was because he was playing by himself on the playground and kids started bothering him so he threw dirt and rocks at them. Then they didn't go away so he was trying to poke them with a paper clip. The second office visit was because he apparently told a kid that he had a knife in his backpack and that if he didn't leave him alone he would cut him with it. Obviously he did NOT have anything of the sort on him or in his bag. This really has me worried. I don't know what to do. I can't afford therapy like the school recommended. He's always so good at home. I've tried punishing   him but he doesn't seem to care. He would rather read and play by himself anyway. Any advice would be greatly appreciated and helpful.

[DeeWill918]

Also, he won't open up to me anymore. Not like he used to. When he gets in trouble he stares at you but it's as if he is looking right through you. His answer for everything is "I don't know". I show him I love him, I play with him and read to him and try to talk to him. Sometimes he even talks in somewhat of a baby or small kid voice and I have to tell him to stop and talk like a big boy. He's teacher thinks he's just acting out for attention. I don't think thats it. If it is it's more than just that.