You are probably correct that there has been some dilution. My co-founder already had very deep pockets before we started the company, and I believe that he remained the primary investor throughout (though I am aware of 1 small round participated in by a venture firm). He would have made it a priority to dilute me out of existence.
They have refused to provide any information with regard to this at all.
There are so many ways to screw over a non-employee, minority common shareholder, you may well have no case here.
For example, if the company hit a rough patch, it might have been recapitalized, with all current shareholders wiped out. Since your partner had 80% of the shares, and plenty of cash, he could have easily pulled this off.
Depending on what state you're in, you may no longer have any right to seek redress. If your partner knows what he's doing (or has a lawyer who does), the best you're going to get is a small amount of cash to go away. And if you guys are on bad terms, you may not even get that.
They have refused to provide any information with regard to this at all.