Hi
My DS has the same problem and have just left uni last summer. As a mum, our first instinct is try to fix the bad situation. I can totally appreciate how you feel. It is not easy to see our children suffer. It's the toughest time we experience as parent.
It looks like your son is bright and still want more for himself. You seem to have a very good relationship with your son and that to me is the key to overcome this trying situation.
I can only imagine how crushing it is for DS to go to uni and found that it is not what they thought it's going to be. The emotions they have to go through are real. The sense of failure, helplessness, hopelessness, sense of letting everyone down and not seeing a future. I have to remind myself not to see the situation as we adult see it (rationally) but from the point of view of a young man's emotion. I'm sure intellectually they know what to do, but emotionally they do not know how to handle it. A very contrary state of mind.
I believe they do want and need to be in control. And doing what they want (like going to the gym) is a form of control.
I also said to my DS that I know he needs time to get over this and asked him how long does he need to get over this (set a date) before we can all sit down properly and discussed the next point of action. And we also ask him how he wants us to help him get through this phase. All these is our attempt to give the control over him to decide and in his own terms. Let it be their choice. And do let DS set a date for anything you agree on. I think this is crucial - as soon as they know they are in control of the situation and not the other way round - they will find motivation to move on.
Perhaps you can try and find out what are the main issues bothering him. Like worries about having no friends, how to fill his times, being oldest at uni, does he wants to go to uni. his views about future etc
We tried to help DS by finding a variety of courses which we think suit him - some related to the course he chose initially and some very different - printed them out and gave them to our DS and let him take his time to look through them. He needed that little push and direction. And then there was a job that came along which we highlighted the advantages of taking it. I didn't think my DS was ready then to take it but just after a week he finds a routine and feel 'useful' again. Interesting choice of word he used. He said he likes being useful. He wants to travel and knowing that he earns it give him a sense of pride too. He has also applied to go back to uni this September. My DS keeps very close contact with all his school and uni friends which is a great great help. He also got himself into a team sport which let him makes new friends. It is still work in progress but hopefully all in the right direction.
So just keep assuring him there are always choices and keep building up his confidence.
Sorry for such a long note. I just feel for you. I'm not sure if they are useful or not. I hope it didn't appear that I am telling you what to do. I just want to share with you our journey with our DS. The time will soon come when he is ready.