That age is hard - hard for grandparents because it is a natural time of separation and pulling away, of activities and <less> time anyway - a time of growing up. It's hard for parents because it's really when - I think - we start to allow our kids some real autonomy and decision making room. And it's hard on kids because just when they start getting a little more freedom - it's also when they start getting more responsibility and more demands.
Kalana- I know this won't make it any easier for you or other grandparents that are hurting - but I hope this helps some. My girls are 12 and 14. My parents and FIL (MIL is deceased) live locally to us. We have good relationships with both. All are retired. My parents travel about 6-7 months of the year. To give you some background and level set. My girls aren't overscheduled. They participate in school based extracurriculars and our church youth group and church based activities. They hang out with their friends. They have home chores. They come from an intact family so only have obligations to one household. DH and I come from intact families and I have one living grandparent remaining - so they have "obligations" to 3 sets of GPs and GGPs respectively. Even still - with all of that - they are more likely to see them - on Christmas and during the summer - and spring break. It's when THEY(my girls) have more free time. There are a few days through out the year that they may see their grandparents and yes they may have dinner with FIL here and there a few more times than you see your grands - but the realities for children that age is that even when you have them right there with you - you are probably less likely to see them often.
So WHAT can you do? What can you do to keep that relationship relevant and healthy? Well, I can only answer for what my mother does to keep in touch with my girls and see if that helps. She's the one they are closest to - because she makes the biggest effort. Does your GC have a Facebook page or is this an option? Both of my girls have FB pages and my mom keeps up with them there. She sends them messages on their walls, sends private messages etc. They play games on FB together. I know a lot of people aren't big fans of kids this age having cell phones or electronics - but my girls have their own cell phones (we have no land lines) and my mother texts them all the time. Before they had their own phones they had texting apps on their iPods and they kept in touch that way. She Facetime chats with them. (Also handy when they visit HER and *I* can talk to them that way) They also have Skype on their laptops and can talk to her that way.. eMail.Twitter.
Then there are the less technological ways. Good, old fashioned greeting cards, snail mail letters, Care packages.
Seeing them face to face is preferable certainly. But you can maintain that relationship throughout the year by those other means and make the very best of the face to face time you have. I know it breaks your heart that time is limited. Just as it breaks your son's heart that his time is limited. It's bittersweet at best when they get to this stage and you realize that all of your hard work is all really going towards pushing them away from you to some extent.