OP it's far to early to be worrying about this.
What you need is a scholarship if you really want him in private school. So you need to research the private schools near you and what they offer.
Quite honestly though, dd was very early at reading writing, spelling etc.
She has done fantastically without private education. She is now in secondary and excelling.
I don't really post about her as some mners can be snippy of you say your child is very intelligent.
You also need to take into account the emotional well being of your child. I am not convince that private school is the best place when you are the poorest kids in the class. I am sure some private schools are brilliant and it's not like that. But how will you pay for all the extras?
What you need to do is research your local schools and visit them. Some will do the 15 hours early years.
Both dd and ds have gone/going through the same primary and it's fantastic.
Don't write off schools without doing your research and visit the schools. Honestly, if your son is gifted it does automatically mean he needs a private education.