My DD goes to a lovely state primary school, but it's a church one so it gets a bit of extra funding from the Church. Plus the PTA is always raising funds for new equipment etc.
A school friend of hers left to go to private primary school after two or three years, but now in Y5 her parents are not sure it was worth it after all as she seems to be learning at pretty much the same rate as my DD.
Small class sizes are the biggest difference which state schools obviously can't compete with, but my DD's school does have a teaching assistant (alongside the teacher obviously) in the younger classes at least, for at least most of the time, and manages to produce generally happy, curious, well-behaved kids, every one of whom the head knows by name. It is a relatively small school and the head teacher is an absolute star, very approachable and fun, but sensible and firm when required.
If you could have a look around local state primary schools, especially Church ones, you may be pleasantly surprised. Though of course it depends on your area. But have a good look around. And then consider joining the Church ASAP if relevant!
Maybe send him to a friendly state school and use any spare money for extra-curricular things such as music lessons, fun language courses, sports, swimming, karate, dance, cookery... develop his interests and fill in any gaps you think the state school isn't filling, as and when they arise, throughout his childhood. If you spend all your money on private school fees (plus extras) then you won't be able to do this. You can start now with toddler classes if you haven't already and if working hours permit - fun for you too!
Also, at private schools each year gets more expensive until you're paying astronomical amounts at sixth form; you don't want to have to pull him out of private school a few years down the line when it really starts to matter. You may be stuck paying for something you can't really keep up with, even if your income goes up. Childhood is expensive enough as it is.
Good luck with your decision, I bet with involved parents like his your son will do well anyway. :-)