Yes, Christina, I do think blended families can be very challenging. I agree, some male children are more distant. As they grow up, many go with their wives and tend to leave their own parents "behind" if there is geographical distances. Yet, none of this explains why your H's children will not travel to see him, especially as H gets older and must bring oxygen with him, etc. It can be hard to understand and may never be sorted out.
The best you can do is to take good care of yourself. Think about how you might want to handle this, the way in which you might handle it, and the timing in which you might approach H to share and to discuss these important matters. If preparing means waiting , then wait for as long as you feel is best..
Much Love ~