Idiom for someone who needs constant babysitting

You can say he's 'high-maintenance'. https://www.macmillanthesaurus.com/high-maintenance

The English language has numerous idioms comparable to 'he would drown in a glass of water'¹. Some examples:

• They're dumber than a bag of hammers (aka dumb as a box of rocks).
  

• They're not the sharpest knife in the drawer.
  

• They're not the brightest light in the harbor.
  

• They're a few screws short of a hardware store.
  

• They're about as sharp as a marble.
  

• Their elevator doesn't go all the way to the top floor.
  

• Their lights are on but nobody's home.
  

• Their IQ would make a good golf score.
  

• They couldn't pour water out of a boot that had instructions on the heel.
  

• Their driveway doesn't quite reach the road.
  

• Their antenna doesn't pick up all the channels.
  

• They're proof that evolution is reversible.
  

• They're running on 3 cylinders.
  

• Their engine is running but nobody is behind the wheel.
  

• If you gave them a penny for their thoughts, you'd get change.
  

• It's hard to believe they beat out 100 million other spermatozoa.
  

• If evolution was an experiment, their ancestors were in the control group.
  

• They would be unarmed in a battle of wits.
  

• They're not rowing with both oars.
  

• Their phone's connected but there's no dial tone.
  

• They're an open book but the pages are blank.
  

• If they had a brain they'd be on the floor playing with it.
  

• If brains were dynamite, they couldn't blow their nose.
  

• If they were any slower they'd need to be watered once a week.
  

• They couldn't hit the floor if they fell on it.

Some of those were shamelessly stolen from (and there's more at) the Not Too Bright list.

¹ _{Your original example would seem to have an international circulation as I've heard it in EN-CA at least.}