Pyeloplasty is a surgical operation that is performed to correct a ureteropelvic junction (UPJ)obstruction. The UPJ is the portion of the collection system that connects the renal pelvis (the larger collecting portion of the kidney that is funnel shaped and tapers down to the ureter) to the ureter. If the transition from the renal pelvis to the ureter is tight the urine will not drain easily and backs up causing dilatation of the collecting system proximal (behind) to that point and enlargement of the renal pelvis . This dilation of the collecting system is called hydronephrosis. UPJ obstruction is usually congenital, but may also be acquired (i.e. secondary to other disease processes or previous instrumentation).
The history of intermittent fevers following pyeloplasty suggests the presence of infection somewhere. It could be either in the urinary tract, around the kidney or in and around the surgical scar. It can be detected by performing urine and blood cultures ( which may be falsely negative if he is taking antibiotics). Also, an ultrasound of the operative site and same side kidney to detect any fluid collection may help detect the presence of any infection pockets where antibiotic may not be reaching. Finally, it is possible that fevers are due to something other than urinary tract and operation.
Since information provided is inadequate, It is recommended that you consult the urologist who operated upon him and thereby is more familiar with his case.