If a lot of money is on the table, all kinds of loopholes can exist. You see stuff like that, all the time, with academic research.
[EDITED TO ADD]
It’s really common for large corporations, with staff counsel, to lodge lawsuits they know they can’t win, because they bet their pockets are deeper, and they think they can force a settlement.
Ugly, but it’s the American way. He may have more luck in the UK, though.
This is a common deal in this industry, and the terms are fairly standardized. They get a small stake, and usually something like follow-on rights, sometimes more. In this case, Pfizer invested further after that. The big drug companies have a very large stake in making sure departing employees aren't scared of taking this particular deal because they write off a ton of drugs this way, and the only way to profitability on that IP is if someone takes the IP and takes the vast majority of the risk and cost, as this guy has. Don't think I've ever heard of an instance where someone took this deal and got screwed by doing so -- only instances where someone didn't (or tried to weasel out of the deal) and then ended up screwed.
> The drug is based on a molecule he discovered while working at Pfizer
tells me that he'd better have good lawyers on speed-dial.