"No I certainly dont worry about him trading me in for a younger model but that's because of a backstory that I'm not going to divulge.
I know that no matter what happens in the future both my DH and his family would care for DS and that is what matters to me most. At the minute I am doing what is best for my DS and the three of us as a family, if we ended up divorced then I have no doubts both myself and my DH would have my DS best interests at heart and we would cross that bridge if we ever came to it."
Oh dear. You'd be crossing the difficult bridge though OP with zero pension, job prospects, earning potential and most of the childcare expectation. That's the whole point. He'd be crossing a bridge where he has more free time, his money is his own, he can afford to set himself up in another place, has the same promotion opportunities he has now or even better.
Do you think many of the 2/3 marriages that end in divorce actually can envisage a time when their spouse would change beyond recognition, treat their wife/husband and kids like shit and resent providing money that the CMS tells them they don't have to?
You're leaving yourself completely at the mercy an unknown future and don't seem to want to acknowledge that. As for the backstory....most people have been cheated on or been hurt/had families and friends hurt through infidelity. They're always adamant they couldn't do it.....until they do.