It is difficult to say without proper examination.
Possibilities can include a lymph node, cancer, hydrocele, cyst, or varicocele. The ultrasound and/or a pelvic CT scan would be a reasonable approach to diagnosing the mass.
A hydrocele is certainly possible in a 2-yr old.
As for whether this is serious or not - one would have to await the results of the imaging studies first.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
kevinmd_b