How are you? An abnormal result of an ultrasound of the scrotum may reveal an absent or undescended testicle, an inflammation problem, testicular torsion, a fluid collection, abnormal blood vessels, or a mass.
But as your urologist has ruled out tumors, hydrocele and varicocele, I feel it could be simple inflammation or the after affect of torsion in the testicle. Reduced size of the testicle after this episode more relates to a testicular atrophy which could be due to torsion.
I would suggest that you take a second opinion in this regard.