- He is picky about food so he only eats what he likes.
- He is picky about food, so he only eats what he likes.
Which one is correct? Do we have to put a comma before 'so' or not?
Punctuation should be understood as being less about "rules" than about "clarity". In this case, the comma is optional. A comma is generally used before a conjunction that links two independent clauses, as is the case here. However the two clauses are quite short and we often omit the comma in this case.
You may use a comma here, if you feel it improves clarity. You may omit it if you feel it is unnecessary.