What PJ said, and what Lydia said.
Personally I would take the UC (urgent care) diagnosis with a grain of salt, because those guys are just trying to treat the cause and get you out the door.
He absolutely needs to f/u with a PCP, and have a full gamut of STD testing, but I say anyone who is sexually active with more than one partner should be tested regularly. If you know what you got and how it's spread it's a lot easier to not spread it.
I also know that mouth lesions and throat lesions can be misdiagnosed from even good PCP's. I personally went through this, with a VERY painful mouth lesion that comes and goes, was diagnosed as a burn, a herpatic lesion, possible oral cancer, cankers, and finally the correct diagnosis.....and why it was so freaking painful.....Shingles. Yup I get shingle lesions in my mouth. They come and go, and I usually get an outbreak when I have been sick, or had some major stressor. Makes the trigeminal nerve path on that side scream with pain.
Have him get some teeth medicine and apply to the spots with a q tip to numb them up. Gargle with salt water solution, and then use a teaspoon of honey after. It will clean and sooth.
If it is a herpes thing he could have picked it up when he was a kid even. Herpes is very very very common anymore. If he gets cold sores, he has it.