My two children were accelerated into K and each started 1st at age 4-5 alongside peers who were 1-3 years older. Their WJ scores were mostly 99+. They both excelled academically for the most part, but they did receive average and even sub-par marks in certain areas that were heavily influenced by their social-emotional and physical immaturity. For example, their underdeveloped fine motor skills made them slower and messier when writing words and sentences or drawing art, and they became fidgety or less attentive towards the end of the school day because they simply got tired (their marks were lower for whichever subject that was taught last). They literally grew out of these challenges over time to meet or exceed grade-level expectations, and keeping them accelerated was the right decision for them.
I suggest that you converse with the school, more specifically the individual teachers for your son, to understand the reasons behind the lagging writing skills and "par" reading levels. The school needs to be more flexible when accelerating children at such a young age - not in terms of compromising academic standards for the grade but rather in terms of accepting short term weaknesses that are mostly or entirely attributable to age rather than aptitude and effort. The bar that the school could consider is whether your son is consistently progressing at a trajectory to overcome those weaknesses in the future. In my opinion half a school year is probably too short to make that assessment, and the 50/50 time spent in K and 1 in the same year seems questionable (unless that separation is by subject) - your son is exposed to two learning models from two different grades, so he could be unintentionally held back by his K peers and teachers from doing better with his 1st peers and teachers.
PS I believe that IQ test results are less reliable predictors at a young age, and that may be the reason why your school did not administer it to your son.
