Daughter has brain tumor and seizures. Her husband visits his son 1 - 4 times a year. He lives in a different town - 3.5 hours away. He pays $50 a month in support. They are not divorced. He has in essence abandoned them. If our daughter dies or has a dibilitating seizure, how to we get custody or guardianship or our grandson?

What do you mean "how"? You go to court and ask for it.

It might be a good idea to as to be appointed alternate guardian now, so that it will be in place if something happens to his mother.

If the father fights it, you may have a problem. I agree with AK, that you should start proceedings now while the mother can make her wishes known.