This is absolutely right, learn before Primary. There are not so many NET teacher in P.1 (Normally only 1 or 2 and 1 or 2 lessons a week for Phonics).
Very lucky, my daughters received a basic Phonics' education during Kindergarten. It's help a lot for English learning.
Phonics would help improve spelling provided that the kid has a reading habit because there are many exceptions in English (e.g. meat vs meet). He needs to know the meaning or else he may write "They a going to the zoo".
On the other hand, if one is interested in DSS/private schools, it would be good preparation for the kid to learn phonics to help him to read even words not yet seen before at school interviews.