The first item that stands out to me is the fairly sizable difference between his VCI (which probably should have had extended norms, with two subtests at SS 19) and his PSI, as well as, to a lesser extent, WMI. Although the gap could just be the GAI/CPI difference often found in GT individuals, it is also possible that it represents some kind of 2e. The relative weaknesses on his WISC-IV are often ones that affect or reflect weaknesses in automaticity, which often manifests in academics as, for example, delays in memorizing math facts, or attaining fluent handwriting/spelling/written expression. With year 5/6 demands, teachers are expecting complete fluency in basic skills, so that mental energy is freed up entirely for higher level problem solving and expression. If he happens to be still struggling with making these skills automatic, the pace and volume of school work will be much more challenging, not because he is conceptually unable to master it, but because he has to "waste" time and energy on nuts and bolts.
Retesting might be a good idea, but not for the reasons the teacher suggested. You may wish to spend some time observing him, especially his basic skills, and any avoidance or other clinically-significant behaviors (e.g., preferentially avoiding extended writing, while readily explaining knowledge orally; going to great lengths to avoid showing work on math problems; requiring a lot of time to complete tasks, but coming up with deep results). Formal assessment may be more informative if it includes in-depth achievement and process measures, in addition to updated cognitive assessment, such as in a learning disabilities or neuropsychological evaluation.
Year 4 is often the year that classroom emphases transition from basic skills development to fluency, preparatory to higher-level problem solving and application. It is possible that this also fed into his difficult year last year.
