So I assumed; and I saw this before responding above.
Welcome back to the forum.
The STD risk is not the same for all female CSWs. Escorts (expensive CSWs by appointment), many of whom are well educated, get regular STD/HIV testing, and consistently have their clients use condoms for vaginal or anal sex probably are lower risk than most non-CSW women. If you choose such partners and consistently use condoms yourself, you'll probably be very safe no matter how often or how long such exposures continue. At the other end of the spectrum, inexpensive street walkers and bar pickups, especially those who don't use condoms (or can be talked out of them, e.g. for higher pay) can be very high risk. Others can obviously have various levels of risk. And geography is important. Far more CSWs have STDs in, say, tropical Africa than in most of Asia. I'm not aware of specific data for Hong Kong, but you could contact national public health authorities for information about it.
1,2) Assuming the frequency and practices you describe in your question, there will be some small STD risk from unprotected oral sex. Avoiding even oral sex will lower the risk still further. You can assume you will be infected with HPV (probably already have been), but HPV is unavoidable and occurs in all sexually active people -- and most infections remain asymptomatic and harmless. Assuming no condom breakage, there will be essentially no risk of HIV, and low risks for HSV-2. HSV-1 risk will be higher, due to oral exposure -- but still low.
3) Sorry, I can't give specific estimates on these questions. We don't always ask most STD clinic patients whether or not they used condoms for exposures preceding STD diagnosis. Your "cumulative" risk of HPV is probably 100%, but it's far, far lower for all others.
My advice is this: Accept that you simply cannot reduce the STD risk to zero, given the lifestyle you describe. Condoms fail, sometimes without notice. So you'll have to accept at least a small risk of all STDs, and therefore I would urge you to have routine STD testing (gonorrhea, chlamydia, syphilis, HIV) every year or so. I would not recommend testing for HSV in the absence of symptoms.
Regards-- HHH, MD
Sorry, doctor, there is one more factor I forgot to mention: he has no other sexual activities besides those described with CSW.