In maths it's significant - but that doesn't prove school will be a disaster; it depends what the school is like. Rather than have him complete a first grade book, I'd be inclined to test him systematically and see what he can and can't do. One option - there are lots of others of course - would be to have him do (for free via a trial period) the initial assessment for ALEKS third grade maths (it does assess skills from the beginning, so this isn't something that would make sense only if you thought he was at that level). Then I'd ask hard questions of any school I was considering.
