The problem is you can't change the pH of your soil, it will be an ongoing effort to keep the soil pH down - you will be using acidifier yearly. You can get a good start by mixing elemental sulfur into the planting hole.
Start by testing the pH of your soil: you will need litmus paper and distilled water. Dig down several inches and take a sample of soil. Put it in a jar (remove sticks, rocks, etc.) and fill with distilled water to just cover the soil. Put the lid on and shake it every few minutes for 30 minutes or so. Drain the soil through a coffee filter and test the liquid with the litmus paper.
It takes about 1 lb. of sulfur to lower the pH 1 point. Also, the sulfur won't have an immediate affect so don't be tempted to add more (too much is just as bad as too little). After the initial plant, you will have to add acidifier (and citrus) food at least yearly, maybe more often.
It may be easier to transplant your lemon tree to a larger pot. Around here, that would be a half wine barrel.
Have you ever checked the soil in the pot on a sunny day to see how warm it is? You may be losing your fruit because the soil stays too hot or gets too dry. The fix is a larger pot and/or an insulated pot. You can insulate a large pot by putting a layer of bubble wrap insulation on the inside of the pot before you fill it with soil. Or by using heavy light colored pots.
How many gallons is the current pot? How often do you fertilize and at what strength? What are you using? How do you determine when to water? These are probably questions I should have asked at the beginning.
