hard knot near to left testis with no pain

There's no telling whether or not it is serious unless he visits the doctor, I had the same problem and it just turned out to be a small cyst on it that was no reason for concern. But this was only proven after a sonogram. Tell him to keep an eye on it for a few days and if there's no improvement then best bet is to schedule an appointment with a urologist, it might be embarrassing but its their job to deal with these on a daily basis.

I noticed a small round nodule in my left testicle three years ago, 0.5 mm in size. The ultrasound scan result revealed growth to 1.75 mm. What can this be?