Welcome to the forum. Thanks for your questions. They have been asked many times on the forum. In addition to my reply now, below are links to three other discussions that address these issues in more detail.
"What is the protocol...?" and "What is the moral thing to do?" This is an issue of personal preference without hard answers. I'll just say that telling partners or not has no effect at all on their risk of catching or having HPV in either the near-term or long-term future, or on the risk they'll have any significant HPV related health problem someday. The choice to tell or not should be considered primarily a relationship issue, not one of disease prevention.
[Is] HPV "always contagious" and "Can HPV be transmitted to a partner after several years of negative paps?" This sometimes happens, but rarely. HPV DNA may persist indefinitely, but the immune system normally keeps it suppressed so that it cannot be transmitted and doesn't cause future disease.
"What can you say to a partner to reassure him and give him accurate information?" The main messages of importance are that HPV is nearly universal and unavoidable in sexually active persons; that most infections cause no important health problem and are cleared up without ever being diagnosed; that the most important outcomes that matter can largely be prevented by immunization; and that nobody should permit an impersonal bit of DNA wrapped in protein, which happened to evolve to exploit human intimacy for its own propagation, to seriously interfere with romance and rewarding sex.
Take a look at these other discussions. You can also use the forum's search function and enter terms like "HPV disclosure" or "HPV and informing partners".
http://www.medhelp.org/posts/STDs/Handsfield-6-month-disclosure-guidelines/show/552283
http://www.medhelp.org/posts/STDs/hpv-QA/show/742564
http://www.medhelp.org/posts/STDs/HPV-without-sex/show/1799277
I hope this has helped. Best wishes-- HHH, MD