With two different IQ tests showing a highly gifted child I would rule out any concept of overestimation. I would consider that one grade skip may not have been enough. This happened to our DD in year 4. She lost interest and stopped reading. She was in no way challenged. It was only part of the problem in a difficult year but it is hard to come back from that. With a supportive teacher and a better environment it will but just make sure he is actually being challenged.
There are some good books on underachievement that may be useful to review to see if they fit your situation. I read them when we had the same issue. They are all available on Amazon Kindle.
Why Bright Kids get Poor Grades by Sylvia Rimm
The Underachieving Gifted Child by Del Siegle
When Gifted Kids Don't Have All the Answers by Galbraith and Delisle
We also checked for vision problems, general causes of fatigue, 2 E issues etc.
The things that worked for us were a full time gifted class initially and a second grade skip. We now have a very happy DD soon to start year 12.
We also redid the SB V and it came out even higher than when she was initially tested so by all means retest. It may help you advocate with the teacher if that helps her understand his needs. She might also benefit from looking at some of the above books, if you think she might be receptive, so she can see that gifted kids may not look gifted if the challenge level isn't there.
Sounds like your DH may have had similar issues at school!
