You need to be very clear about what you can and will do to help him obtain a summer job, while detailing what is required of him in this job-application process.
The best way to show him that he always has you in his life is to offer him encouraging words on a continual basis, reaffirming his good character traits, his intelligence, his sense of humor, and his natural abilities. Expecting him to thank you for your belief in him and his abilities may be unrealistic. He will hear what you're saying and it will have a very positive, comforting effect on his well-being.
Parents need to realize that they've been raising their kids throughout adolescence to prepare them for this very time -- the day they leave home. Talking to other family members and friends about your mixed emotions regarding his leaving for college will help. When both my kids left for college, I had the blues for a while. So many people were coming up to me and saying, "Carleton, you must be dancing in the streets to have your kids out of the house." My reply was, "No, I miss them." It's not unusual to miss someone that you love and that especially includes missing a child who leaves home for college.
If your goal right now is to abide with your son throughout this confusing time, by letting him know that he's loved, you'll be giving him the best gift parents can give their children.