If both of you are willing to enjoy the sex, texting, and generally wonderful times now, at the cost of potential loss and mourning of the connection later, that is a choice you each get to make. Depending on where he's at in life and what he feels ready to pursue, he may not be interested in looking for a person to have and raise children with yet.
There are moments in leadership when no one is watching but everything is at stake. Not because a policy is in question or a metric is missing, but because our moral compass is being tested in the quiet. In these moments, we do not lean on politics or public opinion. We ought to lean on what we believe to be true and on moral principles that will benefit the community we serve.
Tell your husband. He probably won't enjoy hearing this news, but it would be strange-and probably weigh heavily on you-to keep it a secret from him. And openness with your spouse is more important than secret-keeping for your mother-in-law. Then let him decide whether to tell his dad. He'll have to consider all the usual things, like whether your father-in-law is likely to accept the information or explain it away and whether this is the right moment, given his delicate health.