Maybe he's a bit nervous. Is he a new teacher? A common effect of nerves is to rush, not letting the music really get into your ear and body.

The problem should improve as he gets more experience. If he gets off time while you're working with him, make sure he knows it and get him back on track (gently, so he doesn't get more nervous about it, and doesn't look bad in front of any students who may be present).

It sounds like the problem isn't finding the beat, but finding/keeping it while teaching. If it isn't due to nerves, maybe it's having to talk or think about the student's dancing during his own dancing. Again, this will get better over time, but in the meantime (if that's the reason) you could suggest that he not talk while dancing in the lesson. Dance, analyze, then stop and explain. Or it could be that he's letting the student's dancing throw him off. You could have your trainees practice dancing with a partner exhibiting typical beginner problems and see if he particularly has trouble handling it :D.

Failing that, you could pull him aside and kindly say that you've noticed his usually good timing isn't as right on during lessons, and has he noticed that or know why it might be?

Good luck!