Agree with the swing and Scottish snap method.

Play it backwards, extend the scale in either direction--do anything you can to get them thinking or engaged on a deeper level. Count-tap-clap, say the notes, whatever you can think of. Do 1/4 time, 1/2 time, single time, etc. The human brain likes something about those divisions/multiples of the tempo.

If there are others in your band that have trouble with 16ths, do some sort of 2-minute rhythm or subdivision drill everyday. Maybe it's a more widespread problem?? You know them best.