For any new employee (and you don't need to get hung up on the titles), they should generally get an equity grant that is proportional to the value you intend for them to bring to the business.
If you can't grow the business to the next level without this person, they get a stock grant similar to yours. If they are going to implement some "really important stuff" but generally not be involved in the business management they'll get somewhere between "quite a bit" and "a pretty good chunk".
Generally speaking, it's not a good idea to allocate 100% of the stock. You should think up front about what other employees you'll need to hire, what you might have to allocate to investors and so forth and make grants accordingly now.
Even with further information, it'll ultimately have to be what both of you are comfortable with.
Granted, you've done a lot of hard work single-handedly but do not underestimate the value/importance of implementation, especially in a startup environment.
Also, if you all are going to need/raise funding, you should give some consideration to the currently prevailing norms of founder - co-founder ownership patterns acceptable to the funding sources and keep that in mind while deciding on this.
HTH
However, since you're not seeking funding at this stage, I'd say it is pretty much your own call - between the two of you. Have a frank discussion, try to work out what value you've built so far and what he brings to the table and go from there. I'd say anything, 80-20 / 70-30 is okay, as long as both of you are comfortable with it.
HTH
That said, one way to think about splits is to look at the number of hours you've invested, assign a dollar figure to those hours, and treat it like you've made a capital investment and use that as a basis for determining/justifying a split.