Suddenly enlarged testicle in 7 year old

I was just getting my kids in the bath and noticed that my 7 year old son's testicles looked odd. On closer inspection, one of them is hugely enlarged - like about 5 times the normal size. You can see and feel one testicle that is normal but you can't feel or see the other testicle - it's just a big swollen lump. It's not red. He says it doesn't hurt. He is peeing normally. He doesn't recall getting hit or kicked in that particular spot Is this something to be urgently worried about? If it's like this in the morning I will take him to the doctor, but should I be doing anything more urgent? I tried calling NHS direct and the message said they were super busy and were only dealing with urgent cases. Not sure this qualifies. But don't want to just ignore it either. He says he doesn't remember if it was like this earlier I dont think it was like this yesterday. Help!