That sounds remarkably like a principal I worked for a few years ago. Unpleasant man.
Anyway, first thing I'd do is look at any written policies your school has for its gifted program. Do they list missing deadlines as a reason for being removed from the program? (Oh, I guess it goes without saying, but find out whether your son actually DID miss those deadlines, by how much, and under what circumstances.)
"Insubordination" is treated with varying degrees of seriousness depending on the school and people involved, but I've never heard of it being used as cause for denying special education services. Combined with your "vendetta" theory, this suggests that the principal might have a personal issue with your son rather than/as well as with you.
As others have said, get all the documents together that you can.
Ultimately, if the principal is against you, there's little-to-nothing that can be done at the school level; you'll have to approach the district. This might actually work in your favor, though, as this principal sounds like the cowardly sort (from the fact that he didn't make any contact with you about these supposed issues until the last minute). Principals who are that timid in dealing with parents are also usually timid in dealing with their superiors.
