Musicmouse,
I feel so sorry for you. But please don't despair just yet.
5 years ago my DH and I stopped TTC after my 3rd miscarriage. I was 43. We were in a very difficult situation financially, I had lost 2 consecutive jobs as a result of taking time off sick, and the emotional roller coaster was simply too much to cope with. It was a terribly sad decision to make, but we are still together.
DH loves my DS unconditionally and is a great step-dad. Like your DH, he sometimes feels "sidelined", especially when DS is spending time with his natural father, but he also tells me he counts his blessings every day because so many people are unable to have children.
Do you really believe that this is the ultimate deal-breaker for him? Would he destroy an otherwise happy marriage for the sake of a purely hypothetical child with a purely hypothetical future partner, when he already HAS a real live family - you and your DCs?
I'm sorry if this sounds blunt, but the chances of you two having a baby together at your age were always going to be slim - 50:50 at best. I guess your DH had never thought this through (him being a bloke and all) but he really SHOULD have done, and the realisation that he might not have a biological child of his own has probably hit him very hard.
To be honest, I doubt if he wants to split up, and doesn't realise the effect his words have had on you. He needs time to work this through on his own. Give him some time to grieve.
HTH X