We had our sons' name down at a prep school from age 2, but decided in the end to send him to our local 'good' state school.
Planning ahead, we have a grammar school within a mile of our house, the single sex comprehensive is dire. For secondary we would ideally like him to go to the local grammar, otherwise we are likely to pay privately for secondary. There is a secondary academy out of area, which could be an option too.
I took some advice from a few people who suggested we were best 'saving our money for secondary' rather than pay privately the whole way through. I think someone else has suggested this above.
Small class sizes are appealing, but there are risks around prep schools not having to use qualified teachers and teaching methods can be out of date. Academic study can often be overly focused on prepping for entrance exams, and that would restrict his broader learning.
My son is in year 1 now and he's academically thriving in the state system. School is challenging him and we do extra bits and pieces with him at home e.g. nurturing his love of maths, teaching him coding and encouraging him to read as much as possible. We already get plenty of homework, and I don't think I'd want even more that would have come from a prep environment.
Re: 11 +, we will plan to generally support him at home and he'll have a paid tutor in year 4/5 if at that point we still feel its the right option for him. I've now settled on the view, that if he has a state education plus a decent batch of private tutoring and doesn't pass the 11+, grammar education isn't right for him.
I know the system/options are different in each area, so it is worth thinking a little more about your longer term schooling options. Does he 'need' to go to a prep school to get into the 11+ system in your area, or can you support it in other ways?
I really wouldn't want to pull my child out of private and into state at a later point. You're most likely to have a restricted choice in terms of state options in that scenario, so you may be much better to take your state place now. Given the Covid circumstances, I would want to be rock solid with my finances. We have two professional salaries, but I don't think I'd want to have to commit to 7 years of prep school in this climate. I'd also be concerned about the financial viability of the school if it's small and others are starting to pull out.