He is correct. The highest possible score is the same regardless of which grade the child took the test in. However, the 2-5 test does create some "ceiling effect" because it starts students at lower level questions and, as such, requires more "perfect test taking skills" to score significantly above grade level. In either case, I wouldn't be terribly concerned. Any score beyond 255, as you mentioned, is indicative of strong testing taking skills rather than increasing reading comprehension ability. To give you an idea, take a look at this chart comparing various reading level systems: https://resources.njgifted.org/reading_levels.html

I have more information on MAP testing here: https://resources.njgifted.org/purely-gifted/understanding-nwea-maprit-scores