Hi Kay. Even though I only have 1 child who just turned a year, I do have a response to this. My sister and I are 12 years apart. I am the oldest. I spent 2 years at the community college and then went away to school. When my parents took me my sister came with... she thought it was a big adventure. However, when it was time to leave .... things didn't go so well. She started to cry as soon as she realized I was not going with them. We have always been close. I always made time for her and we had our special 'sister days'. Well, she cried for the next hour in hte car until my father finally pulled over, told her to get a hold of herself and everything would be alright.
My mom called me that night and told me what happened. I was heartbroken too! I knew we were close, but it didnt' hit me how hard my going away would be on her until then. So, I made it a point to call her on a regular basis, like once a week, write her letters, and when the school had their sibling weekend I invited her up. It worked out really well. My parents brought her to school and then they had a weekend to themselves and would normally find a cute little hotel or B&B and have a little get a way then pick her up on the way home.
We always made a big deal about her coming to stay with me for that weekend, so it was something to look forward to. When I would go home I would make a point to spend time with her and let her know I still loved her and she was special to me.
I would talk to your daughter and see if she would be open to doing some of these things. He misses having his sister around, her presence, being able to look up to her. I even bought my sister a sweatshirt from my college... she wore it all the time! ha ha ha But my suggestion would be to call your daughter and talk with her about it. I'm sure she will have ideas of her own. Also, at home you can encourage your son to write his sister letters, or send an e-mail, draw a picture for her ... whatever.
I can say from experience, the age difference between my sister and I (although no planned that way) was great. When she was going through some tough times with my parents during her teen years, I was there to help mediate. She would turn to me when she didn't feel like she could turn to my parents and I could help her see where my parents were coming from. Now, she is older and married and we still have an awesome relationship. The age difference doesn't seem so great now and we are still incredibly close.
Your son will adjust to his 'new life'. It will just take some time. In the meantime, just allow him some outlets to show his sister how much he loves her and misses her.