At my uni he would absolutely meet the criteria for Mitigating Circumstances (mcs). These require evidence from a professional (the therapist would count). He could also get Reasonable adjustments for mental health, as required. However at this stage I would suggest focusing on MCs. For students who are suffering from severe anxiety/stress, which he sounds like his, it is possible to defer coursework/exams and/or his final project. This can mean some modules are dealt with in the expected time, and others are postponed to the refer/defer period (for us it is July but for others it is end of the summer).
That can enable some students who are struggling with their mental health to spread out the assessment requirements.
The key thing to know is that universities will normally have a ‘fit to sit’ policy, which means if you submit something, it is assumed to be representative of what the student can do. We want our students to be submitting the best work they can. The majority will be in that position and if your DS isn’t because he is trying to manage issues others don’t have (stress/anxiety/mental health), then it is absolutely fair and reasonable that he takes that extra time. Sometimes students worry that it is cheating or an unfair advantage and it is absolutely not, if he is trying to manage his mental health and his studies all at once. Do also tell him that he has gotten this far, and if he needs to spread out his final assessments in order to perform at his best, then that extra time makes no difference in the grand scale of life, and he should allow himself that additional time.
If you are highly concerned, and you think he needs to suspend, then that is also likely to be possible.
It is so hard to seek help when you are feeling in a bad place, but I 100% reiterate what others have said, get him to talk to his personal tutor or the senior tutor in his department, because they can help guide him on what he needs to do.