Todd Chrisley -- I Don't Want to Drug Test My Son ... I HAVE TO!

Todd Chrisley isn't joking around on his reality show "Chrisley Knows Best" -- he really does drug test his son ... calling it "a necessity."

Our photog in New York City caught up with Chrisley and the rest of the family, where we picked his brain on good parenting ... and what he thinks is the biggest problem facing kids today.

Chrisley has said on his show he's spent a million bucks on rehab facilities for his 23-year-old son Kyle ... but now tells us why Kyle has to be drug tested weekly.

Watch our clip ... Todd makes it clear the testing's not just for cameras.