Whilst he has every right to change his mind, his reasons seem creaky, and the fact that he's only told you this once you raised the subject and are now looking at a timeline suggest that he's known for some time he doesn't really want more DC, but was happy with you assuming it would happen one day, and he'd worry about it then.
He is very unlikely to change his mind - I think that is a way to soften the blow and stop you leaving.
So you have to assume he will never want children. And your decision is staying with him v having children. And as a pp said, staying with him has its own risks that it might not last forever anyway, and you could be left wishing you'd left sooner when you had a chance of having children. You are likely to feel resentment which will increase the chances of the r'ship not working out longterm.
And I would understand the resentment. He appears to have changed his mind but not informed you, and has been happy for you to continue to look after his own children whilst knowing that by stringing you along he was stopping you having any of your own. Despite knowing you wanted them.
At best, if you accept he's only changed his mind very recently, you still have the choice to make. And if he is a decent person he will completely understand that you have every right to leave and make the life you want.
And to the pp who impied you're in the wrong / selfish because your DP should be 'enough'..... I suspect most people on these boards would agree that if you want to have children, looking after someone else's one week end a fortnight just isn't the same.