Eric's 日本語　(Japanese) to English and Back again

So EVERY night I can muster up the strength I read to Eric one of three books. Where the wild things are. One fish. Two fish or my least favorite ABCs with Mickey Mouse. Since we all started living together again Eric's English has improved allot, but after reading these stories to him I constantly wondered how much does he really understand. Last night I found out. 100% I was blown away when I asked him to read the story back to me....... in Japanese and he was able to do it with almost no hesitation at all. When I read some parts from "Where the Wild Things Are". I get pretty into that and tend to be pretty animated about the whole thing. Particularly "And when he came to the place where the wild things are the ROARED their terrible ROARS and GNASHED their terrible TEETH and ROOOLLLLEDD their terrible eyes and SHOWED!!! their terrible CLAAAAAAWWWWWSSSS! Eric's reading was right on! Although I did not know all the words he uses I could tell by the tone and inflection of his voice what parts he was reading. It was truly a great moment in my life.