This is a medical question, but here goes.
I am sure that this was explained at least to your dad. He more than likely had chemo and/or radiation therapy which is toxic to the kidneys.
He is going into renal failure it seems. The rough equation is for every doubling of creatinine your kidney fuction decreases by 50%. In normal kidney function the creatinine level would be 0.6 to 1.2 milligrams(mg) per deciliter for a male. Usually less in older than younger and so on, but a level of 3.8 is indicative of renal failure.
The table below is a very rough measurement. A Urine measurement for Creatinine and Urea needs to be done for an accurate GFR(Glomerular Filtration Rate).
Creatinine Level % of Function 1.0 100% 2.0 50% 4.0 25% 8.0 12.5%
Dialysis is usually started when people are less than 10-15%. Earlier for the elderly and diabetics, etc..
I would think by now that he has been referred to a Nephrologist.
Your dad, as you know, has a bad illness with a very low survival rate. I wish him well and you too!