Since his kidney function is very poor (eGFR less than 15), the breathlessness is more likely due to fluid overload or acid build-up in the body because of kidney failure. In my opinion, he should be started on dialysis, and to support this we need tests like ABG to check acidosis, chest X-ray for fluid in lungs, electrolytes especially potassium and bicarbonate, urine output monitoring, and an echo to assess heart function.