You need to have a long talk with your DH and explain to him why having another DC is so important to you, and that you feel you would never be able to accept not having another DC.
It is hard when couples aren't on the same page, but certainly not insurmountable.
Explain that you fear you are quickly running out of time to have another child (due to how long it took to conceive previously) and that the longer the delay, the higher the chance of complications/ birth defects etc.
Ask him WHY he doesn't feel ready. What is it that he's waiting for ?
You need to be very honest about your thoughts and concerns, and how you would feel about your DH if you do not have another DC. (obviously, he also needs to be very honest too)
OP's have said that whatever you do, it's a gamble and it's true.
You just need to weigh up whether having another DC (or at least trying to) is a deal breaker for you (i.e would you regret it forever).
If it is, you need to tell your husband, and then be prepared for the consequences.