It's hard enough when one parent is dying. When both of them are, it can be overwhelming. If you husband isn't much of a talker, he's not likely to get talkative over this. But the feelings are there. He's expressing his feelings, even if he's expressing them in an astoundingly businesslike way.
Your approach could be - equally businesslike (at least, it may seem so for you!) - "What can we do for them?" This is a way of telling him, "I care about your parents and I care about you."
Sometimes you might try asking him, again in a businesslike way, "How do you think your parents feel about (you fill in the blank)?" This could be less threatening to him; you're not requiring him to get all emotional but to talk instead about his parents' emotions. He may answer or he may not. But the fact that you asked will be significant, because you can be sure he'll think about your question!
Do what *you* can to let your ILs know you love them, even though your style is different from your husband's. You might have your children write little notes or draw pictures for their grandparents and put them in the mail. Let him know when this happens, when you go to visit them, or when you've just returned. Don't make a big deal of it; just be informative. This is probably the way he can handle things right now.
My father-in-law's health started deteriorating last winter. We live far away, so my husband had to make a couple of extra trips across country and I held down the fort at home (if he had asked me to go with him, I would have). He also had to take over a lot of the legal and business matters that come with illness and dying. So there have been time and travel costs, and his family is in the dysfunctional category so there are, um, those challenges as well. But doing this work was his his way of saying, "I care about my parents."
When his father died two months ago, my husband coped by wanting to be in charge of every bit of the planning for getting back there. The way I helped him was to let him do it and be available to do anything he needed. I felt a little as if I were being shunted aside, but it wasn't really so; he's not a person who can work too well with others as partners, and it was better for him to be in charge than not take action at all. And it worked out fine. It was just the ol' being-businesslike thing again.
At the moment he has some "stock answers" for anyone who asks about his dad or his family. It will be some time before he will be able really to talk about the whole thing, if at all. However, I can read many of his feelings without his having to converse about them, and I'm sure you can do the same with your husband. (In fact, to me they sound as if they might be related.)