I don't think he is as gifted as his older sister, but he is clearly more advanced than his peers in pre-school.
His pre-school director use to administer the test and she agreed that the report is not consistent with what she sees.
A few thoughts here:
I've seen both the scores be off due to a variety of thing and I've also seen families where there are very large differences in the intelligence of the members. I've more often seen some people be a lot lower than seen a gifted person who is born into a family where there are no other gifties. Actually, I'm not sure if I've ever seen the latter.
In regard to the part of your last quote that I pasted above, though, is he attending a preschool where most of the children are gifted? If not, it is likely that even a 98th percentile child is going to appear clearly more advanced than his classmates. In regard to the preschool director's comment, I really am less and less sure the longer I interact with my own kids and other gifted kids that one can differentiate btwn HG+ and MG with a driven personality solely by interacting with the kid. I do think that a knowledgeable person can often distinguish btwn bright/high achiever and gifted by interacting with the kid, but once you get into 98th percentile and up, pinpointing the actual IQ based on performance or interaction with the kid is a bit more iffy, IHMO of course.
From what I understand, the SB-V isn't that long of a test. Actually, I think that it may be less onerous for your ds in that it has routing tests at the beginning that help the tester move past questions that are too easy for the child and thus shorten the time of the test for a child who is likely to do well. I hope that you get the score he needs to be placed in the same school as his sister.