It's hard to understand why the school says your son is too old for testing. If a teacher expressed a concern that he has a disability, that should be sufficient reason to test him. It may be that your son is doing so well in other areas that the school feels the spelling problem is a minor issue. However, it's important to find out if his spelling difficulties are due to:

1. Inadequate teaching or not enough time spent learning spelling when he was younger,
2. Exposure to a "whole language" approach to reading and writing that encouraged "inventive" spelling, but never got to the edit/correct/ and "learn it the right way now" stage, or
3. An underlying learning disability.

Some children can do a great job with phonetic analysis (sounding out words), but if they have poor visual memories, they can't retain an image of a word long enough for it to spin down into long term storage. As a result, they don't say (as proficient spellers do), "Hey, that doesn't look right!" He has no model with which to compare it. Reading a lot helps to expose children to words that are correctly spelled, but if a child has a poor visual memory, even that won't help very much.

If your son is tested (which I think is a good idea, by the way), the evaluator might want to use a test called the Rey-Osterrieth Complex Figure Drawing. Your son would first copy a rather detailed design (to evaluate his eye-hand coordination and his organizational skills, among other things) and then he'd be asked to draw the design again from memory. And (you probably shouldn't tell him this) he'll be asked to draw it again twenty minutes later. The psychologist (probably a neuropsychologist in this case) would be able to assess how well he retains visual images. This will give some clues to your son's spelling problems. The neuropsychologist should also do an analysis of the types of errors your son makes in order to look for patterns that will also hold clues to his approach to spelling. If your son is found to have a learning disability that explains his spelling difficulties, then he is eligible (now and later) for reasonable accommodations, such as being able to use an electronic dictionary or a word processor with a spell check feature. Also, his teachers will be required to grade content apart from spelling and to not penalize your son because he has a disability. If there is no disability, then you and your son's teachers need to come up with a remedial plan based on the analysis of his learning style which should be generated from the neuropsychological evaluation. If your son has no learning impairment, but just doesn't care about spelling, then he's just going to have to accept that his grades in some courses (and his job prospects) may suffer in the future. Incidentally, there is a great book about LD kids who are also gifted. It is called Crossover Children by Dr. Marlene Bireley.