Sounds like you have a big mess on your hands. I'm impressed that you've been able to operate without his share of the revenue. How much longer will the company be able to operate without these funds? 

Your partner is giving you a wonderful opportunity--a chance to grow and practice a critical skill. Successful people know how to have difficult conversations. It's time to call a meeting and have a real talk. Continuing to avoid this talk may equate to professional suicide.

Here are the steps to follow:

Prepare. Make some notes about the situation and your feelings. Write about where you are, where you want to be, and how you might get there.  

Come to the table and stay there. Your partner will come to the table if your message is. "I want to find a solution that works for both of us."   

Set the stage. Sit down at a time when you are both clear-headed and able to give this important conversation the time and energy it deserves.  

Speak from the heart. Do not point fingers of blame. Instead, focus on collaboration and finding solutions that works for both of you. 

Define the emotions. It sounds to me as if the situation has set you up
to feel disrespected and devalued. Your partner needs to understand the
emotional consequences of his actions.  Conflicts around money are
often metaphors for feeling devalued.

Listen, listen, listen. Listen as if you are an outside observer with no prior knowledge of the situation. Twenty years in the mediation business has taught me that there are at least two sides to every story. You may be very surprised when you hear the rest of the story. 

Give yourselves time to think, process the information and cool down.

Consider hiring a professional to help you with the collections.

Consider hiring a professional mediator to facilitate your conversation.  Often an outsider can shed light on the blind spots and help clarify an agreement agreement.

Any agreement you reach must be put into writing and signed.  Your
agreement needs to have consequences in the event of default.  Your
agreement should address how you will follow-up and check-up on your
progress.

All the best,
Elinor