Seriously, take @Charlie McCurry's advice to find an experienced and competent real estate/business attorney. I was in your situation (minus the apartments) about 7 years ago - best investment I ever made was in finding my then (and still current attorney). The Operating Agreement between me (through my LLC) and my partner took about 5 months of back and forth to devise, mostly because we were unfamiliar with each other. My partner contributed his land, and found his own 'silent' capital partners - so the attorneys had to negotiate some defensive clauses to ensure each of our 'sides' were protected.
Since he is contributing the land, but you are contributing actual capital, consider having your lawyer negotiate a clause where you both are 50/50 voting partners (and both guarantors to the loan), but you get all the income distribution (minus what your partner requires for annual taxes) until your capital is repaid; you can even specify who gets what percentage of the tax depreciation credit. You'll want to think out what relationship you guys want after your capital is returned in full - but the easiest is a 50/50 split on income and depreciation going forward.
Our situation was actually a bit more complicated as we refinanced with cash out and paid out his non-voting capital partners (bank interest was cheaper for us than their entitled interest going forward); but again, the lawyer negotiated operating agreement made this relatively painless.