You say that your son has difficulties with simple computations. Since kids with ADHD have a hard time focusing or paying attention, he may not be able to stay "tuned in" to the math well enough to process it. Think about it this way: If you are trying to catch butterflies, it's easy if you've got a big net and a few big butterflies. For your son, doing math may be having a tiny net and a lot of little butterflies. Real hard to catch them (and keep them). This isn't really a math disability; this is the consequence of not being able to focus or notice when that plus sign turns into a minus. He also may lack what's called one to one correspondence. He has to understand that the number one equals the quantity one and so forth. Until he does, the numbers make no real sense and he gets more and more confused. He needs to be taught by using manipulatives -- objects that he can see, touch, and move, like blocks, toothpicks, or stones. He may need a dose of special education to develop a sense of mastery at math, and now is the time to do it. The longer you wait, the bigger the problem, and it's likely to leave a bad taste in his mouth for math.
You also say that your son struggles with word problems. You didn't say how his reading was. If he has problem reading storybooks, he's likely to have problems on word problems, even if he understands the math involved. If he reads well, but doesn't process what he reads, he probably has poor comprehension -- and this applies to those word problems as well. Also, if he has ADHD, he probably has difficulty just focusing on what's important and what's not. He may need a reading teacher, not a math teacher.
If you have your son re-tested, ask the evaluator these questions: