Clearly it would help your son if he could find the words to articulate his depressed and anxious state. You suggest that he is becoming more distressed as the days pass and that all areas of his life are being negatively affected. If in fact his distress is a result of missing his brother so much, I would strongly recommend that you reconnect these brothers as soon as possible. I don't know how easily that is accomplished, given your older son's distance from home. But even if your college boy is far from home, I would still set their reconnecting in person as a top priority at this time. Your college son needs to know how his brother has responded since his departure. Perhaps he will know how best to help his younger brother cope with his absence. You need to be careful not to make your older son feel guilty about his brother's falling apart and you need to be certain that you do not make your younger son feel ashamed or like "a baby" because of his emotional tailspin. Make every effort to keep your younger son feeling connected to his older brother through emails, phone calls, and visits to college if possible.
Don't expect the mere passage of time to heal the sense of loss your 15-year-old is feeling. If more connecting to his brother does not begin to improve his sense of well-being and if his life continues to worsen in all areas, I would suggest professional counseling. Let's first see what can be done with lots of empathy and support from you and a strong effort to keep him as connected to his brother as possible.