I've never heard of a CTO having more equity than the CEO. Do you think you're being slightly unreasonable in your expectations? 50/50 feels quite generous after they raised the round of funding.
In signing up for a 50/50 split, you made your decision about this already. If you don't feel like your co-founder is pulling their weight, maybe figure out a way to change the division of responsibilities so that he is instead of approaching this like a zero sum situation.