Though not immediate concern as long as one kidney is working .I would be asking for a second opinion if I was you.Not just taking what sonographer says.

My son has kidney reflux and it wasn't diagnosed inutero but they still treat it seriously and are checking his kidney function and he is under specialist care.Sometimes abnormalities present as blockage especially in boys.

I understand from two other people whose babies had potential kidney abnormailites they needed extra scans before birth as well as checkup at birth u/s after birth just to monitor it.One was referred to specialist immediately after birth and later on had surgery .The other baby's problem resolved.

Goodluck I hope it all goes well for you.