5 months is a long time to be prototyping. You need feedback from real users not co-founders. Who else has seen the prototype ? Perhaps you can gently turn th

5 months is a long time to be prototyping. You need feedback from real users not co-founders. Who else has seen the prototype ? Perhaps you can gently turn this around by introducing the concept of the MVP and estimating how close you are to it. If you can negotiate everyones perception of the MVP down to what you already have, then the next task would be to show it to potential customers. So your co-founder needs to be aware that you're only going to be concerned about real feedback from real customers. This would benefit you all.