I know exactly what you mean. I have 2 children (boy and girl) and it was exciting when they were off to college but sad too.... especially when my baby girl went out to the big world.
When you have been used to "caring" for another person each and every day of their life... it is tough to all of a sudden have it stop.
There are a few things I encourage you to do:
1) Volunteer at a non-for-profit organization. It will help channel the need to help others.
2) Make a concerted effort to discover the new you. You have worked hard to help your son achieve what he has. It is, indeed, a blessing to have your children grow and to become responsible adults with a desire to make their impact to the world. Celebrate your achievement and acknowledge to yourself.... "job well done!".
3) Just because he has ventured on to a new chapter in his life, he will definitely still need you!!!! Be there for him. I'm not sure how far he is from you... but I know that he would appreciate "care" packets from you every now and then (especially during finals).
It is painful to let our babies go.... but we have to... for their benefit... and ours. There is nothing easy about it. But it will be worth it.
BTW... My baby girl just graduated from college last December with a bachelor of science degree in business administration. ;-)
Sending hugs to you!