My son's right testicle has been swollen since last night. It seems to be tender and painful and I am not sure what to do.
You need to take the child to the ER because medical conditions that would cause the testicles to swell are sometimes medical emergencies that require surgery.
A painful, swollen testicle is a call for an ER visit. There, he can be examined to decide if it is an infection or a surgical emergency. If the testis is twisted (torsion), he could lose it.