Urge him to go to the Health clinic on campus. They see this often and will be able to help him. Discuss with him what he wants to do as far as staying in school. It may be very important that he stay in school, stay with the same routine while getting therapy. The college health clinic can help him get into counseling which will help him decide what is best for him.
Do remember that there are different forms of depression. Often feeling down is an expected way to feel when becoming an adult or really any time in life. I remember being depressed around midterms and finals because I felt like I didn't know the material well enough. Then when I passed with good grades the depression lifted.
What helps in that situation is to have support from your friends and family. It may be enough for you to reassure him that he can do well after talking with him about why he's feeling depressed. Or it may help for you to suggest that he see a counselor. He can ask for help from his academic advisor if he's having negative feelings about his academic work. He can also ask his advisor for help knowing how to use the college system to get other kinds of help such as finding a counselor. I suggest that the college will have resources to help in this situation.
You definitely do not want to take over and make decisions for him. Talk with him, yes. Ask him if he wants advice and guide him if he's willing. But don't try to make him do anything with which he's not in agreement unless he expresses suicidal thoughts.
Ask him outright if he is suicidal. Ask him how he's feeling and if he knows why. Ask him what he thinks would help. Offer him ways in which you can help. Work with him.
Do NOT call the school. Consider how embarrassing that would be. Unless of course he states he wants to die and has started to make plans or you see evidence of this. Has he talked about ways to die? Has he talked about what to do with his belongings? Has he stopped going to classes and cut himself off from friends? Those are reasons to involve authorities.
Later after rereading your post and the ETA. It sounds like your son is handling this well. Just be there for him to talk with if he wants to talk and to remind him how much you love him. There is nothing that you can do that would be helpful. He's an adult and will make the decisions.