My wife is Swedish/Finnish. I am an American mutt.

According to the literature, there are two main methods. One is to speak one language at home and the other in public. The other is for one parent to speak one language and the other speak the other.

My wife learned to speak both Swedish and Finnish by the second method as a child. Her father spoke Swedish and her mother spoke Finnish. She also spent summers with her grandmother, who spoke only Finnish. This caused problems when the children for a while refused to speak Finnish with their mother.

We seem to be blending the methods. Our daughter speaks Swedish with her mother at home. Our daughter asked that they not speak Swedish in public. It made her self conscious. Some times I join the conversations in Swedish, though I am far from fluent. We try to stay in the language in which the conversation begins.

I think you do want to separate the two languages, so the child switches vocabularies and grammar rules instead of mixing them.

Good luck! Tell your son he is lucky to to learn two languages. It stretches the brain!