Did the teacher not like him complaining or was it the way it was done, did you ask your son what happened for his input, have a talk with him ,as this was an isolated incident I am wondering that if he normally shy at school what happened to trigger this, give your son the benefit of the doubt. The time frame you say your son stated to have behavioral issues was soon after you had your other child so possibly there have been jealousy issues ,this is very common and you could give him plenty of extra one to one positive attention, outside sports and games, ask his Dad to be involved ,very often Dads work and they dont get to have fun with their children it is important to make sure Dad does do sports and Games with them,Good Luck