All three of my kids were/are in French immersion from Pre-K to mid-elementary (eldest through 4th grade, middle through 2nd, youngest just started 3rd). I don't think that it hurt their English skills. Of course, from what your son reported, he is still well above grade level, just perhaps not quite as much as you expected. As others have noted, this is just one test - another test a few months from now might look quite different.
I don't recall all of the test scores for my older ones, but in 3rd grade, my eldest took both the Terra Nova and the standardized test that kids in France take. She did well on both, and scored in the 98th percentile on the Terra Nova reading portion, 97th percentile overall. On the French test, she didn't do quite as well, but her scores were around 90th percentile. She is now a senior in HS, with high scores in the SAT Critical Reading and ACT Reading. My youngest doesn't have any standardized test results yet, and the middle one had a Lexile range from junior in college to graduate level (take that with a grain of salt) on testing last year in 8th grade.
Don't worry about it - he is fine.
