I would suggest redirecting them to the guidance documents, which indicate that they should switch him to the 6+ in September, so that they can accurately use it as a growth measure during the whole of next school year.

https://www.nwea.org/content/uploads/2017/08/MAP-Growth-Administration-Guidance-25AUG17.pdf

https://community.nwea.org/docs/DOC-2713
(this one is actually on K-2 vs 2-5, but includes points about exposure to content)

https://www.nwea.org/content/uploads/2017/10/NE-MAP-Growth-Administrative-Guidance.pdf
This is Nebraska-specific guidance, but has a bit more discussion on fitting the test level to the instructional level.

https://community.nwea.org/docs/DOC-2842
Comments specifically about gifted students moving from 2-5 to 6+.

...pronounced like the long vowel and first letter of the alphabet...