Hi BKD, I have read that testing young can give unpredictable results, however if your feelings on the wppsi are strong, if you've personally seen him handling this more advanced math, etc. I would go more with what you are seeing rather than results that don't jibe with that.
I think the reduction in his 'apparent' ability in school could be the result of 'hiding', which I've read can occur if a child is trying to fit in, but it's not a good thing obviously. Partly because the child can begin to think that these abilities are 'bad', 'odd', etc., and have a pretty poor self image after a while. And yes, this might spill over to what he admits to knowing at home. Further, you might be running into perfectionism, with the very serious, meticulous mapping out of a particular assignment, or, lastly he might be trying his darndest to make it interesting.
I think you can read more about these on hoagies gifted or around this forum, too.
What to do now is probably the big question; are there other school options?
Further testing might need to wait until you sort out what might be going on in your ds' head. We have tried after schooling to bring back my ds love of math, after encountering some issues with that at school, and it can make a difference. Helping a child see themselves as good rather than needing to change can also be helpful.
Very best of luck with this!
