Sounds like you don't have an equity plan yet - I'd start by proposing one to him. Propose a 50/50 split, but between technical and non-technical, then explain that if you need to add an additional non-techincal founder it will have to come out of his 50%.

This conversation can lead a number of good places - 1) he gets motivated and things go back to the way they were at the beginning, 2) he agrees to add a third person and split his share, 3) he decides to walk away, perhaps with a small share of equity for work already done.