Re-consider the over watering thing and/or get a second opinion. I find that when someone 's biggest problem is over or under watering and you tell them, they have a hard time believing it at first. Anyway, let's talk about boron just in case.

Early Boron toxicity on citrus ususally looks like this:
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It starts on older leaves. Then you can get some gumming on the undersides of leaves and, if it's really high, dieback. But, as stated above, that could also look a lot like other deficiencies.

Is this a new problem? If the tree has struggled in that spot all along then it could be boron. If the tree has been in for a few years and this just started showing up, it is highly unlikely to be a boron toxicity issue unless boron was recently added.

You should have either a leaf or soil analysis run. The soil would be the cheapest way to go. Citrus can tolerate about 0.80 parts per million boron or less. Over 1 ppm in the soil and they get real unhappy. Make sure your sample is a composite of multiple sub samples collected from throughout the drip line of your tree.

The sulfur is to decrease the pH of the soil and make the boron more soluble so it leaches out more readily. But at the same time, making boron more soluble makes it more available to the plant and increases boron toxicity problems until it is leached out. Whether your want that to happen or not depends on what the pH of your soil already is, the level of boron, the lime content and how likely it is you can actually get water to move through the soil profile. That being said, as bpgreen stated, adding sulfur won't do much unless it's incorporated, which would tear up the roots of an existing tree.

The rule of thumb on boron leaching is that it takes about 3 times as much water to decrease boron as it does salts. The rule of thumb with salts in a clay soil is that you have to apply about 3 inches of water to cut salts in half, so to cut your boron in half, you need to apply 9 inches of water. Your boron level will give you an estimate of how much leaching to do (if any), but only an estimate. Keep in mind that proper leaching is awfully tough in a clay soil.

But, if you get your soil tested and know exactly what's going on and it turns out that you do need to leach, you will have to allow the soil to dry slightly throught the root zone (not just the top few inches) between irrigations to avoid creating anaerobic soil conditions and/or an environment that favors root diseases. Leaching, especially in heavy soils, is a long slow process.