If it's just you and the original founder, sounds like you're almost taking the role of a co-founder or cto. If that's the case, 2% does you absolutely no justice. But again, that entirely depends on what skills you have and what your primary workload is.
Another factor to consider is where you're located geographically. If this is in Kentucky and you don't want to relocate elsewhere, 2% is a much better offer than if you were in the Valley. Simple demand/supply.