5 weeks post radical nephrectomy, for my 40 year old son . his egfr is 53 creatine 1.6. are numbers like that expected?

If kidney function was normal before the surgery and CT or US of the kidneys did not see much difference between the 2 kidneys, serum creatinine should be back to the baseline number after 3 months. Creatinine 1.6 is abnormal if your son's kidney function was normal preop. But the other kidney still has time to compensate. If after 3 months, GFR is still less than 60, he will be staged at CKD 3.

These values may improve as time goes on(weeks or months) if his remaining kidney is perfectly healthy. So many metabolic changes occur in response to major surgery and they can put a strain on kidney function. Nephrectomy itself causes an immediate drop in kidney function caused by the loss.of one kidney but compensation eventually develops to a significant degree, maybe not 100%,though.