Since your husband has had one kidney removed, he should have a decline in his total kidney function reflecting the loss of one-half of his kidney function. This can lead to a rise in his serum creatinine. This may be normal for him with a single kidney. A creatinine of 1.3 milligrams per deciliter (mg/dl) is about what I would expect for a single kidney. For patients with a single kidney, I recommend a DASH diet. You can review the DASH diet at: https://www.nhlbi.nih.gov/health/health-topics/topics/dash
I recommend that your husband have blood and urine testing for protein, blood and infection annually. This should be done by his regular primary care physician. He does qualify as chronic kidney disease (CKD) only because he has had one kidney removed, but this does not mean he will progress with his CKD. I suggest continued monitoring and if there is progression, then referral to a nephrologist may be necessary.