The only advice I can offer, is in ANY relationship, even in a D/s relastionship, there has to be respect and limits. Seeing as you your being Married, even with your husbands knowledge, I would hope that your Master would have enough respect for you to know your limits and if you tell him no more childern he will accept that. if he is unable to or unwilling to as harsh as this may sound,, it may be time for you to look for a Master, who you can serve, but still respects your limits, as mentioned above, fantasy and reality are 2 different things
Also, say you did have his child, and you both decide to go different ways, what happens to the childs mother?? any child needs to parents, my feelings would be keep your relationship with your Master as it is now, and draw the line when it comes to a child with him, to many risks involved in having a child with him, more then anything the wel fare of th child as they grow up and you and your Master are no longer together for what ever the reason(s)
As they say 10 minutes worth of pleasre results in a lifetime of commitment and you already have 1 child with his biological father
Keep this D/s relationship as it is, don't add a child into it, it wil only complicate thing down the line