As someone mentioned below, Khan Academy (online and free) has a good reputation; however, for a kid this young, it may not hold his attention.
Yes, you and the teacher need to start over and build a better working relationship. You say you "cannot discuss all this with his teacher" but truly you must discuss it - and tell her that he is now getting nervous and reluctant about school and is fearing math, not liking it as he once did. This is pretty normal when math gets harder at around third and fourth grades but you and she need to try again. Be sure you make an appointment to see her and do not take him--see her alone. Don't be emotional. Find out if she has times when she can offer extra math help -- some schools have periods when teachers can indeed do things like that, or you can arrange after-school time, etc. If she just cannot provide any extra time to see him one on one, ask what she suggests you do.
I also second the post referring to the issues in teaching him yourself. If the teacher gives you materials and works with YOU on working with him, and she shows you how she wants things done, that's fine, but just going on your own to teach him math as you were taught it could backfire.
My husband's college degree is in mathematics, but the terms and methods used today are NOT the same ones most of us learned as kids. The techniques my child was taught in school to use were alien to my husband the mathematician! (He's also from a country other than the U.S. and where he grew up there were even more differences in the terms and methods used.) So because you weren't taught under the same system your son's expected to know, plus you come from another country where the system and terms may have been different yet again from what is used in the U.S., I really would suggest you not try to teach him yoursellf (unless it's specific stuff the teacher has given you to work on).
Like Gamma G, I am not saying you can't work this math -- just that the way you teach your son to do things may not be the way the teacher will require him to do it on a test. He may have the right answer at the end, doing it the way you teach him, but if his workings on the problem do not show the steps the teacher wants, he will hear about it and possibly lose points.
Our daughter (now 13) has done a tutoring program through Mathnasium in the summers and they also have tutoring during the school year. I would consider whether your son might benefit and gain confidence by working with a tutor or tutoring place like that in your town. It's not about his having any real lack of understanding -- it's about building his confidence so he doesn't get nervous and fall behind, because math moves FAST these days and kids are expected to keep up or they'll be behind the next year and it snowballs.
Tutoring is expensive but it is a good investment in his confidence. One iimportant thing -- a good tutor will know the school's curriculum and teach him the way the teacher and curriculum want kids taught, in your particular system, right now. A casual tutor like a high school kid on his or her own might not have that advantage, so I'd go with someplace that can guarantee your son would get what's on the curriculum.