This sounds like it's ripe for the you-cut-I-pick process. Ask him how much equity he would give up in order to have the CTO title. Then after he says how much

This sounds like it's ripe for the you-cut-I-pick process. Ask him how much equity he would give up in order to have the CTO title. Then after he says how much he thinks the title is worth, you get to pick whether you give up the equity (to him) and get to be CTO, or whether he gets to be CTO but you get a piece of his equity.