I even thought about taking off work when there is a test to completely explain the question to him. He doesn't like to tell the teacher or aide that he doesn't completely understand the question.
The teachers who work with your son need to create an environment that will make it possible for him to do the very best job he can, even if he has trouble taking a test. This might involve reading the questions to him. Teachers might also read the directions out loud to the entire class and then have your son (and maybe some other students who need this approach) do the first item on the test. Then the teacher should check his work to see if he understands the process involved. If he's on the right track, he can continue; if he's confused, the teacher can intervene at that point to give him suggestions about how to do the work correctly.
Unless students have this kind of guided instruction, they may fail -- not because they don't know the information, but because they don't know how to communicate what they know.
Special education teachers (or parents) who work closely with students taking or preparing for tests have been accused of "feeding" the answers to a child when they work with them. For example, they may say: "The first president of the U.S. was...? G..G...G...George. George what? George Wah, Wah, Wah. You know, what you do with a TON of dirty clothes." This approach simply fills in the holes in a child's knowledge with a teacher's ideas. It's more important to find ways that allow a student with LD to demonstrate his or her knowledge or skill in the way that they can.
For example, if a child uses clay to create a model of the first president crossing the Delaware, we have an idea that the student really knows something about this important historical event. Asking a student with language-based learning disabilities to write a two page essay won't show what he knows, but how poor a writer he is. This is simply a bad test of a person's ability.
Right now, your son seems to lack the self-confidence to tell his teachers he doesn't understand a question. He needs to keep working (with his teacher's help) on this important skill. In the meantime, it's the teacher's job to find out when he's having difficulty with a particular item, and to help him get over the hurdle.