Some books will also have "RL: __" printed inside the front few pages. RL 4.2 would be 4th grade, 2nd month, for instance. That may give you some idea of his reading level.

Their reasoning for not testing beyond a certain point doesn't seem legit to me. My girls were always able to keep testing until they couldn't do anymore and we were certainly given #s well above the grade in which they were currently enrolled.

Study Strategies for Accelerated Learners