My son from Romania does this when he has an ear infection(he has no symptoms of an ear infection, only oppositional behavior).
More likely, he is experiencing some attachment issues. Although very frustrating, since he has had a year of good behavior, he likely will bounce back pretty quickly with some attachment parenting and therapy. Start by rocking him 20 or 30 minutes a day. Also, try not to react to the negative behavior. Just give him a consequence(or let the school if it's school related) and don't eaplain it because he already knows why. If he sees it's bothering you, he may do it more to get a reaction. If you allow it to be his problem, it isn't any fun for him to continue. You might want to check
www.radzebra.org ,
www.nancythomasparenting.org , ww.attach.org .
The latter two should have listings of therapists in different areas that can help you.