I don't really understand. He doesn't want to wear any protection with the girls, but he wants to minimize his exposure? Good luck. The only 'standard' sex act more risky vis-a-vis STDs for a heterosexual man than unprotected FS is unprotected anal sex.
The short answer is: Don't have sex with promiscuous women (hookers or girls you meet at bars) if you aren't willing to deal with the risk.
If he wears protection for FS but does the DFK and BBBJ, he's reducing his risk quite a bit, but any sort of fluid exchange is going to have the possibility of some risk. I can't say for sure and I might be talking out my ass, but I don't think there's as strict of testing standards for Japanese porn stars vs, say, the strict ones for American stars -- most mainstream JP movies use condoms, where American ones at this point don't.
I think most girls do the provider thing after they've retired from porn, although there are exceptions to every rule- it would certainly be interesting to see how often the legal provider businesses have their girls get tested, considering they aren't supposed to be having vaginal sex in the first place.
In general I don't think Japan has quite the public health setup that other sexually liberalized countries like, say, Germany or the Netherlands have. The sex industry is decriminalized and borderline legal, but the local and national governments very likely don't want to shine a spotlight on them in order to inspect what's going on with STDs.