Generally speaking, the Lexile measure has value in the context of multiple measures. Many schools use it as a screening, progress monitoring, and instructional grouping instrument, but I would not recommend its use for high-stakes decisions (certainly not in isolation).
This gives a rough idea of what Lexiles translate into:
http://teacher.scholastic.com/products/guidedreading/leveling_chart.htmas does this, which has two charts, one for the average student at each grade level (what typical kids can actually do), and one for instructional expectations (what we ask typical kids to do):
https://lexile.com/about-lexile/grade-equivalent/grade-equivalent-chart/or this:
http://teacher.qa.scholastic.com/pr...s/SRC_GradeProf_Brochure_final_email.pdfNotice that, on the last and most fine-grained of the three charts, your DC's Lexile places him in the middle of the proficient range for grade 12. Given that, there is a sense in which a new Lexile measure would not provide additional information of value, since he is already well above grade level. Although Lexiles can extend much higher than that (I've seen scores above 2000), he may have reached the technical limits of the measurement instrument being used by your school for his grade level.
