Try to involve someone with authority. Some angel investor, another co-founder, mentor, advisor...anyone who can hold (both of) you accountable for your efforts

Try to involve someone with authority. Some angel investor, another co-founder, mentor, advisor...anyone who can hold (both of) you accountable for your efforts and actions (or lack of it). You should talk to your co-founder about your concerns. You ll need all your efforts after the launch for product improvement.