Dude, you need to create a Google Now for dick investors.
I assume your issue is that replacing his $$$ isn't easy and you could see other investors back out if the lead investor goes... and so you want to keep the money.
What's the chance that he actually had a reasonable expectation of a board seat? Maybe you were nodding a bit too much and he assumed that was a done deal... then, once you have his money, you come back and tell him you've changed your mind. If you were to be honest with yourself, can you see that? I'm going to hazard a yes guess. And, if so, can you see why he would freak out and feel like he's been had? Maybe he's not the anti-christ.
OTOH, maybe he is. But unless it's a trivial amount of money for him, he has no incentive to pursue low-probability legal action or attempt to scuttle a future financing - both are self-defeating. If you really have a solid case why he shouldn't expect a board seat, then put your head down and get back to work. The most important thing you can do for him and everyone else is to execute.