I would simply tell your husband that he doesn't get to dictate to you how you communicate with your children, and that you will NEVER lie to them -- EVER -- but will always tell them the truth, in an age-appropriate way.
This is good. Send him this everytime he sends you an email. When he gets the same response over and over he will figure out that you are not saying anything more on the topic. Your health is your personal business. Don't discuss it anymore.
