Let him go. I know it may be hard at first but he needs to learn to be independent. I know when I left for college my parents were feeling probably the same way you do now especially since I'm an only child. The truth is it gets easier the next time he leaves. It's a process that takes getting used to but he has to grow up sooner or later
You can go and visit your son in that other state so that he can know that you support him. You can also do things around the house that can keep you busy, if you do not have a day job. Some of those things include gardening, mowing the lawn, taking care of the rest of your family and taking care of yourself.
When this time came for me, I called him so much he got angry. Then I was REALLY a mess. What worked for me at first was sending him a weekly care package with his favorite homemade cookies, some healthy snacks, and then I would throw in things her might need like fingernail clippers, a little first-aid kit, aspirin for headaches, extra school supplies like electric pencil sharpener and such, and I would spread these things out. Soon the care packages I sent were sent every 2 weeks and then later every 3 weeks. But what I'm saying is at first, the care packages help you work out your thoughts with them being away and at the same time you're still supplying things a mother would in the packages . Also set up a time 3-4 times a week you both have the time to talk about things. He will remember these care packages forever and they will help you work through not seeing him as often. It works. And be prepared for him to get homesick whether you think he will or won't . They will call and say they "feel weird and they don't know what's wrong". That is homesick. Promise them it will pass. You will be very surprised how much they will still need you.