Bryan Fuller, the creator, stated he isn't. Characters speculating on a diagnosis aren't a diagnosis, nor representation. Plus, Will's original comment implies the spectrum he's speaking of is the neurodivergent spectrum (though with a very big misunderstanding on what that even is). The full quote is:

"Where do you fall on the spectrum?"

"My horse is hitched to a post that is closer to Asperger's and autistics than narcissists and sociopaths"

"But you can empathize with narcissists and sociopaths."

"I can empathize with anybody. It's less to do with a personality disorder than an active imagination."

Which is Will denying that he has any of these conditions, but he'd be closest to autism if anything.