If the project has not started yet, the I would say that you would be a co-founder.
For how much of a percentage of the company you should get, I think it depends on how much he has invested and how much he is asking you to invest. Also, if you are putting as much time in this as he/she is, then I would strongly suggest a near 50/50 split. Nothing less than 40/60. Also remember that you may need to give some shares away in the future for other parters or partnering with companies.