12 year old son has lump on testicle

Hi

I was wondering if anyone had experience of this. My son has just turned 12 and he told he found a pea sized lump on his testicle yesterday. It is only painful if he keeps touching it. He is very shy and won’t let me see it. I am planning to take him to the GP ASAP but I’m getting worried in the meantime (obviously not showing him my worry) 

TIA for any advice