So I agree with all the comments about needing a lawyer as to the specifics of your situation.
As far as being unreasonable: The sense I get from reading your narrative is that from your perspective, because you wrote the code for the software, you are responsible for a large proportion of the total enterprise value. I think that in general, engineers tend to discount the contributions of the business side of things. Reading between the lines (and correct me if I'm being uncharitable here), your partner is responsible for the idea and overall business strategy, all capital and expenses(!), marketing, branding, sales, contracting, and customer support. So even though you wrote the code for the product, there's a very large gap between that and creating a competitive and profitable product. He probably views you, with some justification, as more easily replaceable, which is why he perceives you as having less leverage. So while you should certainly try to get as much as you can out of the situation, this description of events doesn't strike me as being particularly unfair to you.
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So I agree with all the comments about needing a lawyer as to the specifics of your situation.
As far as being unreasonable: The sense I get from reading your narrative is that from your perspective, because you wrote the code for the software, you are responsible for a large proportion of the total enterprise value. I think that in general, engineers tend to discount the contributions of the business side of things. Reading between the lines (and correct me if I'm being uncharitable here), your partner is responsible for the idea and overall business strategy, all capital and expenses(!), marketing, branding, sales, contracting, and customer support. So even though you wrote the code for the product, there's a very large gap between that and creating a competitive and profitable product. He probably views you, with some justification, as more easily replaceable, which is why he perceives you as having less leverage. So while you should certainly try to get as much as you can out of the situation, this description of events doesn't strike me as being particularly unfair to you.