A few ideas for you to take, leave or shake your head at:
You can tell your son straight up that you would appreciate a healthier relationship from his end. Keep it real simple and honest and non-judgemental.
You can do the same with your DIL.
You could also cater to (soothe) the (childish) fears of the woman who has your son by the you know whats. Control is often a sign of fear. Probably she needs to be reassured over and over and over and in as many ways as is reasonably possible until she gets the freaking message that you respect her "position" in your son's life, as mother of your grandchild. Build a relationship with her until she finally feels safe and secure with you. Become an asset to her. Treat her like the daughter you need to get to know until she no longer reacts jealously of any time you spend with your son. (I know it seems ridiculous, but that's just how it is with some insecure people, I'm not sure if your DIL is being insecure or just insensitive or a bit of both.) The more you empower her, honestly and unselfishly, the more likely she will learn to respect and appreciate you.
You could also back off and wait until they come to you. This may involve grieving the potential loss of the "normal" relationship illusion you are clinging to.
The next time they want you to be available for them, don't be. Perhaps they will begin respecting your time more when you demonstrate that the "I'm not available" thing is a two-way street and you are not their doormat, nor will you allow them to take advantage of you forever.
Also, stop paying for things. There's nothing less healthy than a relationship that is, in any way shape or form, bought.
You could also let them see you cry. This emotional HONESTY might reach their otherwise icy hearts and give them a glimpse of awareness about the real effects of their behaviours and choices.
Your feelings matter too, grandmama. Stop trying to protect them from the real effects of their decisions. When you do this, you are LYING to them.
Above all else, do what you would do and say what you would say if you knew that you had only a week left to live and there was no more time for bs beating around the bush.
I hope your kid(s) come to appreciate you more.
S.
ps - one more thought ... you could also put your energies into being ready for when your grandchild grows up and has a mind and heart of it's own to decide about a relationship with you. send your unselfish love in little ways and means possible regardless of who tries to stand in the way and trust that they can and very likely will always love you back and better days will eventually come.