Hi there and Welcome
How did you find this out?
My aunt was born with one kidney and she lived to be in her 80's with no problems at all.
If he starts to develop edema,fatigue, stones, then I would be concerned.
He should be monitored through blood test to check his kidney function and occasional scans (ultrasounds, CT Scan) for any changes in the kidney.
Is there a history of kidney disease in your family?
As for it being genetic,anything is possible but as far as I know no one else in my family was born with one kidney we have other kidney issues that were inherited.
Keep us posted.