My daughter was 11 last year when my oldest son went off to college. I taught her how to email her brother. When we take photos she attaches them to emails and sends them to him. Create an email account for your 7 year old. Ask him what he wants to tell his brother and you type it for him in an email. Take photos of what your 7 year old is doing--soccer games, going back to school, doing his homework. Attach the photos to the emails. Pick one or two days a week and send a newsy email from his brother. Then CALL your older child and ask him to send a quick email back.
My younger son was 3 years old last year. When we were talking on the phone with the son at college we would let him talk on the phone to him and that seemed to help.
My youngest child had the hardest time. He didn't understand that his brother was not going to be gone FOREVER. To him waiting for his brother to come home at Thanksgiving seemed like forever. But after the first few times his brother came home and then went back to college he seemed to understand better.
My son that is two years younger has his own cell phone and they still talk all the time.
Mail a care package to your older son who is in college. Involve your 7 year old in the process. Take your 7 year old to the grocery store and let him help you pick out what to put in the care package.
Involve your child in planning for the next time your older son is home to visit. Take him to the grocery store with you. Let him help you pick out what to cook for dinner, what snacks to pick up to have in the house, what to bake as a treat when he is home, etc. Ask him to make a welcome home sign.