I can totally understand your feeling as the same thing happen to my 2.5yo gal, few months back. Mine didn't have fever but just simply refuse to pass urine. Moments before she passes urine, she will scream at the top of her lungs and grabbed her diaper. Good thing is we discover early and it was a surface infection as according to the doc @ Kandang Kerbau Hospital. She recovers after taking antibiotic.
Hope your boy recovers soon. Take care.