Help me!

My son(9) is giving us a lot of heartache. We moved from the US to UK. He seemed excited about it. He started a new school and started to make friends. Well, he seemed to turn and has become a serious problem child. He used to be a good student,but now he gives his teacher a lot of trouble. He used to be a respectful nice child. Now he is speaking inappropriately,kicking other kids and today he spit on a girl. I honestly almost started crying when his teacher told me he did that.  We have tried every kind of discipline(positive and negeative,bribing,etc,etc) except for sending him to military boarding school. His father has spent more time with him and making a real effort to be there. I am honestly clueless,worn out and heartbroken. Could anyone give me any ideas to turn this around. I fear he is going to get kicked out of school and become a hopeless case to his teachers.