This must be very hard for you, particuarly if your DP is playing around with your feelings but I think you need to be honest with him and tell him that you want another child and won't be truly happy or feel fulfilled unless you have one. If he is not willing to have anther child with you then, hard as that may be, IMHO you have to respect that.
I don't know what others think but I think it's a truly dispicable thing to do to trap someone into making you pregnant.
He might be quite happy to have sex with you but ultimately to make you pregnant is his choice - not yours.
What would you think if a man drugged his partner, who considered that they'd completed their family, and impregnanted her because he really wanted another child? I'm sure you'd think it was shocking but it amounts to more or less the same thing and is just as unacceptable as what you're planning to do IMHO. At least that woman would have the option to terminate the pregnancy if she didn't want it. A man wouldn't though would he?
I know there's always the argument "you'd love it once it was here" but a child is not a puppy or a kitten that you can hope will be too cute to resist when push comes to shove. A child is a huge financial and emotional commitment for the rest of your life.
You might have to accept that you and your DP now want different things out of life. When that happens in relationships people have to take stock and consider what they want to do.
If having another child is so importnant to you perhaps you need to think about finding another relationship with someome who does. You are 40 and a half and still have some childbearing years left.
If keeping your family together is importnant to you, then you might have to sacrifice what you want.
It seems that your DP has laid his cards on the table so it's your call at the end of the day.