My son in first grade tested into 5th-6th grade on the math parts of the WJ test. He really has to work on his math facts.
He has a processing learning disability. On the WISC, he got 98-99% (99% on most parts) and 40% on the PSI part. He has written into his IEP for his hearing loss that he can have extended time on tests but he has never used it.
I would just keep working on it. flashcards. I bought Kumon. I have him once a week do the 2 minute drill they do in school.
Interestingly, my son seems able to answer the questions faster verbally than with writing. I've suggested to him that when he does a written test, to whisper the question to himself to see if that accesses some faster, different part of his brain. His handwriting is excellent so it isn't that.
My son is doing Mathblasters but at a lower level- he said it was "stressing him out" at higher levels since it was "too fast."