He receives his 15 spelling words for the week on Monday and is tested on Wednesday. We study the words after he's calmed down -- he gets very upset just saying "spelling words." By Wednesday morning, he can spell all of his words. However, when he has to write down his spelling words for his test he can't manage more than 50 percent. His grades in other subjects are in the 90s. I'm not sure of how to help him. What can you recommend?
If your son learned to spell the words orally, but then he has to write them down for the test, there may be a disconnect. If he can spell the words orally pretty well, ask his teacher to give him his spelling test orally before she gives the written test. At least then she can see that he really knows the words, but that the problem is in output (that is, in the actual writing of the words).
Your son may have a problem with the physical aspects of writing. It might take him so long to write each word that he can't keep up with the dictation and makes mistakes. An analysis of his spelling words and his test results should tell you whether that's a factor. If it is, then he should be given the test more slowly. Try recording the words on tape so your son can play them and stop the tape while he writes the word. (If the teacher is very busy, you might even prepare the tape for her -- the sound of your voice may also encourage your son.)
I would suggest that -- at least initially -- the teacher give your son a grade based on his oral spelling. How about having your son use an in-class computer to produce his spelling words? That would be fun and get around any fine-motor coordination problems he might have. I know the teacher will be concerned about how this will transfer to written spelling, but that's another matter. If he can spell the words orally, and if he gets lots of positive feedback from her in school for this, the transfer will happen more easily. If the teacher is worried about how to grade him, why not suggest that she give him two grades (one for oral and one for written spelling)?
Another reason for the problem you describe is called test anxiety or performance anxiety. Your son may "lose" the ability to spell words he knows because he's nervous about not getting them right. We know that stress increases the negative effects of a learning disability, so we have to change the condition that creates the stress. If someone can give him the test alone, in a quiet room with as much time as he needs, this might help.
Studies also show that people do best on all kinds of tests that are given in the same place where they prepare for the test. You might ask the teacher to temporarily count spelling tests that you give at home on Tuesday night. (You have to promise not to help -- at all!) Most importantly, create an environment that will allow your son to show what he knows and not what he doesn't know. If he's able to spell these words correctly in his writing assignments, that should be sufficient evidence that he knows them. His ability to spell correctly in everyday writing is much more important than passing a spelling test.