I haven’t been to Busan, but I’ve been to a few other areas in South Korea. Traveling isn’t too bad as there are pictures and info booths everywhere. If his girlfriend is there as a guide that makes it that much easier.

I would travel light as sometimes quarters are tight on trains/buses.

I have. I lived in the area for two years, and I visited the city several times. I was a young teen, though, so my information is that of about a decade ago. If you’d like to PM me anyway, feel free!

In general: Koreans are so incredibly foreigner-friendly, there really is not much you need to be concerned about. Your son will probably have a group of helpful locals around as soon as he looks lost for a second. Tell him to say hello to Korea for me. Most of all, the smells.