I would start by having hubby tested for titers. It's extremely rare for someone of our age to not have been exposed at some point. Even if he never developed a full blown case, he could still have some immunity. My brother had two blisters on his back, if I hadn't had such a horrible case, my mom would have just assumed they were bug bites or something. His titer is as high as mine.
If he is not immune, then I would look into dh being vaccinated. Since he could get it from anywhere, not just your son, vaxing your son to protect your husband is not as effective at keeping your dh safe.
Your son's best bet is still to develop natural immunity.
Nine kids and four , living and learning all the time