In over your head; over your head

He's a little over his head right now.

He's in a difficult situation, just like a swimmer in trouble who may have the water 'over his head'.

I've heard the phrase a little different. "He is IN over his head." Meaning a swimmer who is in water over his head.

A similar phrase: "That's over his head." "That's over my head." Meaning that is beyond one's understanding, brain power, area of "expertise," etc.