His left kidney maybe non-functioning since birth as already advised. His right kidney needs to function more than what’s indicated. One possible reason could be dehydration at the time of test. Give him a lot of water, avoid over the counter drugs, and get a ultrasound scan to look for any abnormalities like reflex, Urine tests to look for infection and proteinuria. He needs to have strict bp control and frequent follow up. He has single functioning kidney, which by the dtpa report, is just about normal.

Your  Right kidney is functioning good, probably it is the only functioning  kidney. Left kidney is small one and function is also less. Condition may be since birth. No need to worry as the GFR is good Have to be in regular follow up