Your BATNA in this negotiation is "get on with my life and leave you with zero"; that may be a strong enough position to get your partner to let you buy him out (or vice versa). That's about the best that will come of this.
You could "lawyer up", but that's not going to help you that much, unless your former cofounder is dumb enough to take you to court or make a break for it with your code. It's hard to argue that someone shouldn't talk to a lawyer, so I'll just say: don't spend too much money on it.
There's no chance you're going to work with someone who threatened to sue you because you wouldn't sign over your code after he tried to take a 60% share of a product you coded by yourself.
I'm being grim, and surely that will irritate you or someone else in this thread, but I'm trying to point out the path that costs you the least time, heartache, and money.