Mama I'm learning the voice you're learning

"Mama, I'm learning the voice you're learning," Ivan told me today.
"Sruju." Struja is the Croatian word for electricity (or 'trissity as he says it in English).
"Now you and me need to teach dada. Then dada will teach Allen. No, then mama, dada and me will teach Allen."
"Struju," he repeated.

Even though Ivan fully understands Croatian, he refuses to speak it. He says he can't or don't know how. (And when he was learning to talk, the first words and sentences he'd say were in Croatian. However, as he became more verbal, especially this past year at preschool, English took over.)

I've been quizzing him for months what's a Croatian word for this and that, and the words I ask about are the words just used in a conversation with him. He never wants to tell me. He says he can't. I've tried telling him that if he and I speak Croatian to each other, it's like our own secret language, but he didn't by it. I've tried telling him who else in preschool speaks another language with their mom (Kimia, Andre, David, Eddie, etc....). I noticed that kids whose both parents speak another language at home, will speak that language with them, but if it's only one parent who speaks it, then the kids are more reluctant to speak it.

In any case, I'm tickled that he's expressing interest in it.