What would it take to get the cofounder interested?
Can you realistically discuss these issues without finger pointing and blame?
Could you move on to something else with equal potential?
What is the company worth, realistically, as a going concern, not in terms of what you have invested in it?
If the personal relationship is broken beyond the point which it can be readily repaired, why would you want to move forward?
Good Luck.
explaining this legal situation to him will probably make actually going to an attorney unnecessary
First, try to negotiate a termination of service with your cofounder (talk to an attorney about this).
Second, quit yourself (but again, be honest as to why) and move onto something else.
The above assumes you guys haven't already launched and received traction. While the two options remain, it gets much more complicated to decide if you are launched with traction.
You should tell him that you need more help, and he needs to give up equity so you have something to offer to someone else that actually has time to assist you.
1. Offer a buyout (perhaps a buyout of your stake).
2. Resign and start your own company.
3. Document failure to perform. Consult an attorney.
4. Grin and bear it.
5. (not listed here)
It's amazing how much people don't like to talk to one another about things.
With 4, you just end up getting pissed off and exploding later. 2 and 3 burn bridges, 1 could be conveyed as wanting to take another path if you sell or potentially greedy (he sells)
The other partner has leverage, be it legal, financial, or emotional, and when you're dealing with someone with leverage over you, you need to consult every last option before you even think about bringing things up. I think the OP's right to consider his non-conversational options.