This is most likely a condition called pyelectasis, which means that urine is collecting in your baby's kidney and not passing out into the amniotic fluid like it normally should. It is most commonly seen in boys and is usually harmless as long as his kidneys are otherwise healthy and there's enough amniotic fluid surrounding him (since after the second trimester, the amniotic fluid is mainly composed of baby's urine).
Your doctor will probably continue to monitor the fluid in your son's kidney for the remainder of your pregnancy to ensure that it's not accumulating past a safe level. He may also perform another ultrasound to rule out the possibility of an obstruction, which can be a more serious, but very rare, condition. Most cases of pyelectasis get better after your baby's born and has had a few wet diapers.