sticked!?

A student of mine is in Kyoto with her family. Her son is 5 years old and has been going to kindergarten since about half a year ago. My student says that her son could speak only English before going to kindergarten, but now he speaks Japanese very fluently. On the other hand, he is gradually forgetting English. One remarkable example is that he started to put "ed" for every Enlish verb when he wants to talk in past tense, such as, "I sticked" or "I buyed". The student said it was interesting. I think it is amazing that the little boy still doesn't forget the basic rule to make past tense using "ed".