I am a native English speaker and my wife is a native Spanish speaker and we both speak the others' language fluently. We live in a predominently English-speaking country. We have been raising our two-year-old son to be bilingual where I speak mostly English to him and she speaks mostly Spanish with him. We do however occasionally use the other's language with him and each other but we never mix languages in the same sentence or paragraph.
When reading counting books with my son, he will often say something like this:
uno, dos, tres, four, cinco, seis, seven, eight, nine, ten
First, if we start counting in one language should I correct him when he uses a number from another language?
Second, at what age should we start introducing the concept of two languages and say "Let's count in English" or "Let's count in Spanish"?
He knows some words in only one language and other words in both. For example, he uses airplane/avión interchangeably. He is not stringing too many words together to in sentences yet but I suspect that when he does, he will also start mixing those. What is the best way to address mixing?