I understand where your husband was coming from, but I don't think he's thought it through properly or has seen what has happened to state education over the last decade or so.
My parents put me through a top hot-house prep that got scholarships every year to Winchester/Eton/Radley etc. At the time they had no experience with 11+ or girl's school entry (I was one of the first girls in the school).
In the end I went to a 13+ entry grammar. It was not the super-selective with 90% ex-prep pupils that it is today when I went there. I was bored rigid for the first couple of years, by the end of which my work ethic had worn off - I was fortunate to be bright enough to do well while coasting in the extreme. I also found it very hard to be one of very few private school pupils (pretty much everyone came from the grammar stream at the local comprehensive) and was I was teased (bullied) over it.
With our DD, we are doing it the other way round. Initially I was very worried about living in an area of London with very poor private options and very high levels of deprivation. DD is in a state primary with over 70% FSM and 80% EAL and loving it. They have stacks of extra money, her class has a teacher plus a full-time TA for 30 kids.
They sussed a few weeks ago that she is coasting below her ability level, have moved her into the extended learning group (yes they are teaching at different ability levels in Y1) and are now offering an hour twice a week of free after-school 1-2-1 sessions, not because she is struggling, but to get her right up to speed and work on her dodgy 'work ethic'.
85% of their kids got L5 in English and Maths last year, and 20% got L6 Maths - and that is with a very challenging intake and very few MC parents. They got a child into Eton, 2 into super-selective grammars and 5 into super-selective indies last year. That means that 85% of the children at this inner-city primary are working 2 years ahead of their age. The idea that preps are all so far ahead is imo a myth once you get to Y6. They seem to get them all reading and writing as fast as possible - which may not be a great thing developmentally - but it quickly levels out.
I have been blown away by what I am getting for free.
I do know that I am also very lucky to have such a great school, however, at primary level children have a much greater desire to please and are generally excited about what they do at school. It's secondary where the problems can really set in if you don't have good local provision.
Our local secondaries are DIRE - celebrating 36% getting 5 GCSEs is not my idea of a great school. We are saving our money to aim for a selective indie secondary. IMO the benefits at that age far outweigh the benefits of a private primary.
I would encourage you to look at the state options in your area - not just the Ofsted reports, but actually visit them and see what you think.
A lot of people also start off in a state primary and then move to the private sector either at 8 to prep for 11+, or at 11 to prep for CE - or employ a tutor. That way you can save the fees for many years to put towards secondary and still have the preparation that you need for the private secondaries.
I can understand that you would be loathe to move your children who are at the prep already since they are happy - although imo children are very adaptable and they probably wouldn't find it hard to move into the state system at a young age. But, it seems that this particular prep isn't providing anything much in terms of amazing results or extras and appears to have a very 3rd rate Head (plus, why pick a CE school if you are moving them at 11?)
I would definitely at least start the youngest in the state system. People hate the 'State till Eight' thing and I see their point completely, but it is a valid option. You might find that you prefer the state option. Don't worry about the whole 'if I did it for one then I must do it for all' thing - my 3 siblings and I all had completely different mixes of state and private schools and none of us feel hard-done by.
Sorry, have written a novel here, but I feel for you and think you are in a very difficult and torn position trying to do the best for your children. It's worth considering whether an extra 2 years at home with your son will benefit him more in the long-run than a mediocre sounding prep...
Good luck.