> When I asked my co-founder for rights to use the existing code in my new venture, he refused, citing concerns over sharing IP.

OP seems to imply that their co-founder intends to use much of the existing code base they've developed over the last 6 months for the proposed pivot. If that's the case, it doesn't seem like co-founder would be open to resigning as an employee and director and creating a new code base from scratch, especially considering that the co-founder seems to have control of the existing company.