This is about him, not you or your family or even your husband, and as much as you don't want to hear this, you need to stay out of it unless your help is requested. You can't force him or your MIL to "communicate" if they choose not to.
We lost my Dad to kidney failure and leukemia in February. Dad was diagnosed with the kidney failure 7 years ago, the leukemia a year ago. He did not want to let anyone in to his world of dealing with it, refused any help offered, he "pretended" for nearly 7 years. But it wasn't really pretending, he was grieving the loss of his health, his independence, his mobility, his life...denial is a part of the grief process, some people never get past it, he finally did the last month of his life.
He did finally allow one of my siblings to talk to his doctors to find out what his diagnosis/prognosis was, but only her, and the way the law is now that's the way it is, no authorization, they cannot talk to you, so you can't just show up and expect a doctor to inform you. Right now it seems to be your MIL who has that designation, and if she chooses not to share it's her right, and perhaps his wish.
I know you want to do what you think is best for him, but it isn't your call. And, transplants, trials and such may not even apply in his case, we were told past 60 Dad wouldn't be considered. It is frustrating to sit back and watch what you believe could be handled differently, but unless he is mentally incompetent according to his doctors and a court, he gets to deal with it.
Also, this is important for you to know, unless you or your husband has "power of attorney" that your FIL has signed or signs in the future, you have absolutely no say in your FIL's treatment, it may be your MIL who has that designation, either in writing or because she is his next of kin.
This doesn't mean that you shouldn't try to do as much as possible with him for the memories it will give your sons, you should. We lived with Dad until the day he died and my nephew and little guy have memories of their "Tata," good ones of having fun and laughter with him and not so good ones from the last few weeks he was actively dying. They knew he loved them and he knew they loved him.
And, if you open your family to spending more time with him and you MIL the day may come when they open up a little, and you can say, "if there is anything we can help with, please let us know." Dad didn't accept the help until he needed hospice, other than us fixing his meals, administering his meds, and getting him to the doctor, and his wish was to die at home, so that's what we did, cared for him that last month until he died.
I know this is hard for you, you sound a lot like me, wanting to do whatever is possible to help someone we love. But remember, one day you may be in his place, and it will be your right to deal with it as you choose, you will want and expect others to respect your decisions as well.
My prayers are with you and your family Ƹ̴Ӂ̴Ʒ