From the details provided, your child has a hypoplastic right kidney
with reduced perfusion
This is most likely due to developmental abnormality and nothing much can be done now.
Factors you should take care:
1. Avoid the right kidney getting obstructed or infected. (there's nothing much you can do to avoid, but just be vigilant to identify things early).
2. Get regular USG
to make sure the left kidney is fine.
3. It is possible for the right kidney function to reduce further as the child grows. This is because the left kidney will grow normally while your right won't.
Most of the times, the left kidney will enlarge and take up the extra- work. Just monitor his BP
and if it goes out of range, get him treated.