The past is not a "sunk cost" given the IP issue. It sounds like the poster just continued research the prof was already doing in his lab.

At an extreme, suppose OP just abandoned the prof and started the company solo. Certainly the prof could later sue for his fair share. I would suspect the University (and/or agency the research funding came from) may have some IP rights here too.

Essentially, you can't just treat this as "the start of a business". You need to look at it as "we invented something in the lab, and now we want to sell it".

He can certainly negotiate, but 50/50 does not sound unreasonable. One key component in my mind would be "how much value was created already" versus "how much value will be added in the coming months/years".