It's been my experience that the wife's family gets to see the kids most often. Rather frustrating and disappointing, I think.
I hope you get to see the grandbaby soon.
Do you video chat? You could try making a video of you reading a book out loud. Maybe your son's childhood favorite. Gently inserting yourself via long distance methods into their nuclear family niche might encourage them to involve you more. Emphasis on gently since some DIL are downright prickly.