This must be really tough for you both, TheOne but if your son isn't capable of taking action to apply for an intercalation or Leave of Absence, you need to step in. Prompt action is needed here, really. Depression is an illness think of what you'd do if he's broken an arm quite badly? Then do that.
I'd be finding out the name of his Department's Welfare Tutor, Director of Studies, or Undergraduate Director (different Departments use different names) and email him/her, outlining the bare bones of your son's illness, and asking about next steps for applying for LoA. You can go on a Department website, or ring the Departmental office. If the administrators are like mine, they'll be really helpful, within the law of course.
CC your email to your son's official University email, and say that you're taking action on behalf of your son. Tell the Welfare Tutor (or HoD or whomever) that your son has given you permission to take what action you can, and give the tutor your son's number to call him so he can confirm this. It really sounds as though he's not well enough to deal with it himself. Give a phone number at which you can be reliably contacted (and don't do what one parent did to me a few years ago after they'd practically harassed me about dealing with their child's illness, and then refused to take a call because they were in the Quiet Zone of a train & didn't want to leave their seat because they had work to do. Grrr)
Anyway ...
I think you may also have to have a potentially difficult conversation with your son. In my experience (ack! Over 25 years! When did that happen?) sometimes this kind of behaviour is precipitated, or exacerbated, by a student's own actions: severe underperformance, or a student getting themselves in a tangle over attendance (or rather, absence), late work or work not done, or failing (or expectation of failure). This may not be the case for your son -- it may simply be that he is ill (well, that's not simple, but IYSWIM). But if there are other things that mean that he hasn't been pursuing his studies with what we call "reasonable diligence" you need to know the worst, as if you go in to help with half-knowledge, you may find that tutors are not completely sympathetic. If there is something more than illness, you need then to deal with it as impassively & non-judgementally as possible, which is hard, I know.
Students can go into a real panic and do very silly things over fear of failing or fear of not living up to expectations. I've sat on the highest disciplinary committee of my university to determine penalties for exam cheating etc and one thread throughout the hearings is the student story that not to get a First or 2, i, would be unthinkable. So they cheat, and it is 200 times worse than an honestly earned 2, ii or 3rd ... it's so frustrating and sad.
So he needs to be reassured that if he has got himself into a mess, it can be sorted out. We've seen it all, honestly. Just as an individual tutor, I've dealt with 20 year old alcoholics, rape victims, and students who've done criminal violence (well, the latter we went straight to the police), so his tutors will just want him to be upfront about his difficulties, so they can straighten things out.
Of course, if there are no underlying issues, then it's a matter of medical evidence and advice. As quickly as possible, and I suggest that you take action (and I rarely want to deal with parents, except in cases like this).
Good luck.