Hi,
I'm not sure how useful this will be but can tell you about our experiences. We moved to France (Paris) nearly 9 years ago. My DH was offered a job, I stopped working when we moved (I had a job that I really enjoyed). Two and a half years ago we moved from Paris to Italy, near Rome (DH different job in same organisation). Our DS, now nearly 7 was born in Paris, he was 4 when we moved here.
Moving to Paris wasn't such as big decision as it was something we had always wanted to do. There were only the two of us then and it was a lot easier. Although I loved my job the opportunity was too good to miss. For us, the move from Paris to Italy was probably the same as your move to France, we had a good life there, were very settled, had DS by then, had finally managed to learn to speak French, and the only reason for moving was that DH was fed up at work.
Any move abroad is a bit of a leap into the unknown, even if you know the country you are going to well. There have been lots of threads on here recently about the advantages and disadvantages. You have a huge, huge, advantage in being able to speak the language, the most difficult barrier to enjoying life in your new country (in my experience) doesn't exist for you. I have always loved living abroad so I'm very biased but for me the advantages have always outweighed the disadvantages and difficulties you sometimes encounter.
The best thing about living abroad for me (personally rather than the whole family) is being able to really experience another culture and way of life - really experience it, and understand how another country and the people think, live, work etc., and be a part of it. This all sounds like airy fairy nonsense I know but it is true for me. I also think it has given my DS a great advantage in life - not just being bilingual but the experience of different cultures and ways of life.
For the disadvantages that you are thinking of:
Losing your job - true - but you could find work in France (teaching English perhaps?). Also you may find you enjoy a bit of time with your DS discovering things together - if someone told me a few years ago I would have enjoyed being a SAHM I would have called them barking - but I have enjoyed it and still do. Money considerations are a different thing though - I understand your fears about that - being skint anywhere is not much fun.
DS moving schools - that will be hard on him perhaps, but he will cope. I know lots of kids who have moved countries and they all find a way, most of them are much more resilient and adaptable than we think. But only you know how difficult this will be for him. Personally I think that the advantages for children outweigh any short term problems.
Further away from family - yes - but they can visit, phone etc. It is not that far away. You may end up spending more 'quality' time with them than you do now if they come and visit for a couple of weeks at a time.
Isolation - possibly but as I said above speaking the language is a massive advantage. Having children is great for making friends, and your DH's job will help.
Selling your house - you don't have to, we still haven't sold ours in the UK, you can rent it out.
When we moved to Italy I was really unsure at first. But after lots of thought we did it, mainly for the airy fairy reasons above really - to do it all again in another country. Also that DH was so fed up at work it was starting to affect life outside of work, and there was no chance in the near future of a different job in Paris. I do think though that you both have have to be committed to your new life, a few doubts are normal but major ones would cause problems.
It sounds like this has been a bit of a shock and you really need to have a good think about it, talk it over with DH. He is excited, you are shocked and you need a few days to get over this and have a calm think and talk about it.
Sorry - this has turned into a really rambling reply! If you are still reading I hope it helps a bit
Good luck