Sorry to hear that, most probably he developed another sepsis we don't know the source maybe from lung (commonest) you also told us that he also got kidney shut down due to hypotension he definitely need renal replacement therapy (most probably done and need to do regularly until urine output come again ),Factor that decreased oxygen saturation:
1. sespsis from cardiovascular and respiratory origin
2.Too much fluids in the body due to renal failure or heart failure
3.Kidney unable to remove drugs or other toxins in the body may cause disorientation.
4,Obesity reated causing splinting of diaphragm
If the condition persist he might need non-invasive ventilator but if cant sustain good oxygenation may require intubation again.
Just pray for him hopefully he wont deteriorate.Zain