I should say for the sake of disclosure, that I support gay marriage, and worked quite hard within my professional organization (CAMFT) to change the stance of said organization. It was eventually successful. And, I do see polyamory couples.
With that said, it sounds like your son is happy and hopefully his wives and children are, too. It is not surprising that you would want him to live a "one man one woman" life since you were raised with "one man one woman." Also, while this is changing slowly, it is easier to live in our culture as a "duo" in terms of society's acceptance.
However, not everyone is wired the same.
I think the more important question is whether you are happy with your relationship with your son? Are you getting what you need from him and giving what you want to?
You are entitled to feel uncomfortable. Our feelings are there to let us know we need to attend to something, and usually once we do, the feelings pass. It's okay for you and your husband to think and feel differently about this. In a good marriage, we think together, not alike. Ask your husband to let you feel as you do, and you let him be okay with it.
One way I like to think about important issues is this: I am on my deathbed, reviewing my life. What is important? Will it be how many wives your son has, or whether you feel loved by him and have loved him well?
You may decide to seek counseling for yourself to sort this out inside you. And perhaps after you do that for a while you will find you want to go with your son to counseling for a little while so you can talk with him about what's most important to you.
You may find the book, "Difficult Conversations" helpful, too.
June, let me know if you have follow-up questions.