He is frustrated because his future is now on pause.
He is frustrated because he wanted to buy a home and potentially get married and start TTC and now that timeline has gone out of the window.
He is frustrated that you and DH have both worked in what sounds like very good careers which paid for private schooling but don’t have any spare savings for if anything were to happen to one of you (which it has).
He is frustrated that one of you losing your job was a potential problem (like it is for anyone) and you can’t afford to even pay £400 a month.
He is frustrated that you obviously live well above your means and always have.
He is frustrated that you need to sell your home but haven’t done it yet.
He is frustrated that you are throwing private education and money that you’ve gifted him back in his face and now act like he owes you something.
And most importantly he is frustrated that his mum is very poorly.
His behaviour is completely unacceptable but his frustration is understandable.
What are you and DH doing to resolve the issue?
You say you need to sell your home, it would give him his money back and so why isn’t this being done?
Is it because you don’t want to let go of the big house in the posh neighbourhood?