I was hoping that it was ignorance on his part and that once you confronted him with the info he would act sharpish.
But at both your ages (considering you want DC), and at your stage in relationship (4years in), by this time, 18 months more waiting, is far too long.
I think what he's thinking is....I'll propose to her in 18 months (by which time you'll be 34.5 years old). She won't kick up a fuss before the 18 month mark, because she's too invested in me already, to rock the boat too much, sunken cost fallacy.
Once proposal happens at 34.5, the wedding planning etc, will take another year (35.5 years old). At 35+, successfully TCC, odds are not in your favour.
He's successfully run down your clock. At which point he'll say, 'oh well we tried'. But it's not good enough, because he didn't want to try with you, when the time was viable biologically, for you and him (as PP said, younger men usually create healthier more agile sperm). You'll then resent him. Relationship will fall apart.
If he really loved you OP, he would propose to you in the next month, after you had such a serious conversation with him.
If you meant that much to him, the fear of loosing you, would whip him into proposing quickly, like within one month maximum.
From the sounds of his responses, I would say he's playing you for a fool. Give him until the 29th February. If he hasn't proposed by then, you propose to him. Play him at his own game. Call his bluff. If he says anything other than yes, then you have an answer and can make the decision to split up if you want DC. If he says yes to getting married, book a quick registry office wedding asap, then TTC. In this way, you've given yourself a 2.5 year head start in TTC. Compared to going down the traditional route of waiting for him to propose and having a lavish wedding.
But be warned, even if he agrees to marriage. It's likely that you'll be doing all the work, wedding planning, childcare etc. He just seems the 'laid back' type.