'My son is living in Toronto' > a present state, like 'My son lives in Toronto', but it could be recent move.
I live in Toronto but Mike is living in Kelowna: a sense of contrast between the speaker's more stable home and Mike's recent move. Where the present simple and the present continuous appear in a sentence with a conjunction, then the sense is that Mike's situation is less permanent.
A lot of grammar is about what is already known and unknown and what the speaker believes the other person wants to know.