I could present your case as an absolute classic. Everything you describe is entirely typical for initial oral herpes acquired from someone with recurrent (if asymptomatic) oral herpes.
1) Most oral herpes causes no symptoms. However, among those with symptoms of an initial infection, it is common to have severe sore throat without external sores--even though external lesions (lips, etc) are the primary manifestation of recurrent outbreaks. If the doc is right about the diagnosis, your partner undoubtedly caught it from you.
2) Yes. Asymptomatic recurrences of oral herpes are not as frequent as the same problem in genital HSV-2 infection, bu ti happens.
3) Yes, genital herpes often occurs from oral contact with someone with oral HSV-1 infection. However, nobody gets reinfected with the same virus type. Once your partner is over his current infection, he will be immune to catching it genitally (if that is your concern). The reverse--oral herpes from a partner's genital infection--is much less common, but it can occur.
4) Yes. After the initial episode is over, your partner will be susceptible to recurrent oral herpes (cold sores), just like you used to have.
Oral herpes is often caught by non-sexual, non-intimate contact, just like you experienced all those years ago. Nothing I have said on this forum conflicts with that.
Finally, I hope your BF is on treatment. His infection will resolve much more rapidly with treatment with acyclovir, valacyclovir, or famciclovir. In the meantime, reassure him; in the long run, this isn't a big deal. He just joined the 50% of the population with oral HSV-1 infection and in the long run is unlikely to have any kind of serious problem with it.
Good luck-- HHH, MD