His data are approximately correct. He gives all the sources and assumptions at the bottom of the article as well as a spread sheet with all the data.
Compare his data to Preliminary state data shows drop in rate of elevated lead levels in Flint
In the third quarter of 2010, 8.3 percent of Flint children 6 and younger showed elevated blood lead levels.
The figure decreased to 4.1 percent in the third quarter of 2013.
During the same months in 2014, the figure increased to 7.5 percent and decreased to 6.4 percent in the third quarter of 2015.
3 percent of children children younger than 6 years old and tested since Oct. 1 have had blood lead levels above the federal threshold for lead.
where "elevated" means above 5 micrograms/dL
and to Elevated Blood Lead Levels (EBLL) in Michigan 1998 - 2008
Children less than Six Years of Age, which only shows higher concentration levels.
So in 1998 9.7% of Michigan children under 6 had at least 10 micrograms/dL of lead, whereas at the worst quarter of the current lead problem 7.5% of Flint children had at least 5 micrograms/dL of lead.
Also, in 2005, 16.2% of Flint children under 6 had lead levels over 5 micrograms/dL according to the Michigan Childhood Lead Poisoning Prevention Program 2005 Data Report at page 55 , so even just one decade ago lead levels were more than double that of the peak of the recent lead-level spike in Flint.
Edit: A really good source is Table 1. Incidence of elevated blood lead levels (≥ 5 mcg/dL) among children less than 6 years of age, 2010 – 2016 which includes annual data for each year 2010-2015 for Michigan, Genessee county and specifically for Flint.
The Flint Blood Lead Level data are:
So 2013 was the only year that had a lower level (nominally without considering uncertainty) than the "crisis" year(s).