You describe a very low risk situation for most STDs. To your specific questions:
1) The results of testing for the common STDs (those for which tests are routinely available) are reliable, regardless of the risk level of the person tested. In any case, the sexual lifestyle you describe for your partner is not high risk by any standard. It is low to middling risk for most STDs.
2) Of course I have no way to judge whether his other partners in turn may have had low- or high-risk lifetyles, but your partner's negative tests remain reliable. That fact is more important than trying to guess at things you can never know.
3) Of course he was not tested for HPV. There are no routinely available tests for HPV in men. But HPV isn't worth any worry. We all get it at least one; indeed, probably most of us get more than one infection. HPV is a normal, expected, and natural consequence of human sexuality, and even people with only one lifetime partner often become infected. That's why women need pap smears, regardless of the apparent risk for STD: even the woman at lowest risk for STD has a high chance of catching HPV someday.
The HPV vaccine protects against 4 of the most common types most likely to cause disease, and makes sense for most sexually active younger persons, like yourself. You should discuss immunization with your health care provider. Aside from that, there is little to be done and HPV isn't worth worrying about, since most infections never cause symptoms and clear up without the infected person ever knowing they had the virus.
It is reasonable for all sexually active persons outside mutually monogamous relationships to be tested periodically for common STDs. Although I see little risk in your current partnership, by all means seek testing if the negative results will comfort you. Although some infections (e.g., chlamydia, gonorrhea) would show up within a few days of a new infection, others take longer. But you need not wait long; 4-6 weeks is plenty. But don't expect HPV testing. Rather than worrying about HPV now, just wait until the time comes for your annual pap smear -- and have your next pap smear then. There is no point in any HPV testing other than that.
I hope this helps. Best wishes-- HHH, MD
men are not tested for hpv. if he is a carrier, there is no way for you to know unless he has the low-risk genital warts.
Sorry.....in question "b" I meant to say that she may have been unfaithful to him, he is not sure but suspects.