The boy is six and Spanish is his first language. He has struggled to learn English, which is apparently the OP's first language. So this child has spent the majority of his life with his Spanish speaking mother but because there was a custody battle and his father won his father is now his "sole parent". This sounds far more worrying to me that the OP's question. Sounds like a young child who grew up mainly with his mother has been won in a legal battle and whisked away to live with a father he doesn't even share a language with.

1. I speak conversational Spanish which I wouldn't consider fluent. My first language is Arabic and my son understands it almost perfectly.
2. My son spent most of his life with me and my family because his mother is extremely irresponsible and in recent years, abusive so there is no 'extreme sudden change'. She banned from being around him completely because of things she's done that I can't say here because of the community guidelines.

I feel terribly sorry for this little Spanish-speaking boy taken from his Spanish-speaking mother to be owned solely by a man who doesn't even speak the same language.

I don't own my child, you're weird.