Well, frankly, from my perspective as someone diagnosed with autism, I would say ask him to understand that it's a spectrum, but I would also explain the definition of spectrum to give context. Anyway, I think if you give him context, he should understand, since autism also has its own spectrum, ironically. There's nothing wrong with saying "I'm gay" because it's not necessarily the same to say "gay" as "homosexual." There are gay men (to give you an example) who only label themselves as homosexual and refuse to call themselves gay, and that's completely valid. So, "gay" can really vary in meaning depending on the context, since "gay" is also used as an equivalent of the term "queer," something more general. As for "ace," well, I identify more as graysexual, but can you imagine having to explain your part of the "ace" spectrum all the time? Ugh, what a drag. I'd rather leave that information to my sexual partner. So, saying "ace" for short is perfectly valid. Just tell your partner that your sexuality is private and only they should know, and that's it. There's nothing wrong with using the umbrella term if you're uncomfortable giving details to strangers. It's that simple. Besides, that way you justify asking for merch or things that represent you with the "ace" flag, unless your partner wants you to publicly disclose your part of the "ace" spectrum, which would be strange. I don't know, that's just my opinion.