ETA after your So What Happened:
If what you say is the whole story, then it sounds like you gave TOO much of yourself to him as he was growing up. Let him depend on you too much. Let him think that you owed him being there for him. Let him think that he owed nothing back.
That doesn't mean that you did something wrong or were a bad parent. It means that he didn't learn the lesson of loving back. And now he has dropped what he doesn't need, instead of loving you for who and what you are.
This is a damaged individual and it's not just YOU that he feels this way about.
You didn't say if he has custody of the kids. If the ex has custody, does she dislike you so much that she won't let you see the kids?
I stand by what I said below. Your family members have no right to dump all this on you.
Original:
Your family members are jerks for having said this to you. "Being a grandmother" is NOT making 500 mile trips. And your family members are jerks for enabling this behavior for their own selfish purposes.
Continue sending gifts to your grandkids, but don't make them expensive ones. For all you know, your son isn't actually giving the gifts to the kids. He could be giving them to Goodwill. Don't send him gifts anymore - he doesn't deserve them and you should not be rewarding his behavior with trying to "buy" his love. Don't try so hard with him.
I honestly think you should not be close to these family members who treat you like this is your fault because of not traveling.
There's not enough information in your post to make a guess as to why your son is acting like this. You haven't mentioned your relationship with his ex-wife. Who has custody of these children? You haven't talked about what precipitated you "needing a life" with the circumstances of your move. I'm not going to say that you have been a model mother because I don't know. But if you cannot fix this with your older son, you need to do some soul-searching as to what it was that bothered him about you so that you don't repeat it with your younger son. And not making 500 mile trips is NOT a reason for an adult kid to throw away a parent...