How do I ask my boss for money he owes me?

When I was hired at my employer my immediate boss agreed to an arrangement he had made in order to intice me to take the position. Since I was already currently employed locally and the new position was an hour commute each way he then agreed to give me an additional weekly income on top of my salary. Please note that I would not have taken the position if it had not been for this arrangement because without this arrangement I would be taking a pay decrease. The additional funds were to be used toward lunch, gas, cell phone expenses as well as a temporary Petty Cash account to purchase office supplies, postage supplies and things of that nature. He in turn still reimburses me for office supplies eventually. I make several monthly trips to the post office and drop-off’s to other companies as well as use my cell phone to make calls or send text messages to him and other office associates on work related subjects. In the beginning he paid me upfront cash weekly, then as the months went on he began to pay me later and later. First it was every 2 weeks then 3 and now lately he has only been paying me at the end of the month. This past week will be the end of the 5th week without receiving any money… I know he always pays me eventually but when my bills are due mid month and I can’t get by with just my salary checks I have to borrow money from my boyfreind in order to keep up on my bills…On top of that I still continue to make special trips and he still expects me to purchase supplies up front as needed to continue business. I told myself if he did not pay me by the end of the week I would need to sit down and talk to him, he has been out of the office so I sent him an email to his iphone asking him if we could sit down early next week to discuss some things that have been on my mind. I am having a hard time figuring out how I should approach this issue since it is an agreement between only me and him outside the company. Sorry for the long explaination but as you can see it is complicated, how do I start this conversation and come off as non threatening and without giving him an ultimatum?