Their biggest concerns may surround how family life will change without him there. You can offer them at least a practical outline of what will change and what will not. I think most of the comfort and consolation that can be provided will come from big brother himself. If he has had good relationships with his siblings, he can spend time with them in various groupings and in one-on-one special times. During these times he can reassure them all of his love for them. I know this may be a formidable task for him to pull off but its the best way to preserve these bonds.
Talk a lot throughout the summer of how you will all stay in touch. He should be made to feel that he is going off on a grand adventure, not that hes leaving mournful family members behind. There will be all the normal attendant anxieties of a first born leaving the nest. Use the summer months to celebrate yourselves as a family that cares about each other, that can laugh together, and that can weather any separation.
Congratulations on your first college child. Take it from one whos been there with my two kids, the time will fly by. My kids both loved getting letters (and now e-mails too) in addition to the occasional phone call.
For more on how to say good-bye to your college-bound kids read Carleton Kendrick's Parents Say Goodbye: The Last Summer Before College