he was fallen from the roof but he couldn't reach to the ground
cuz he was caught on the brench of the tree.
so u can say it's not completely fallen.
it's a play on words.
덜 means deficient,not complete
떨어지다 means fall
and 덜떨어지다 has own meaning by itself.
it means there is some dirt on his head like a children.
means still thinking or doing like a child and not completely grown yet.
they use this word(덜떨어지다(덜떨어진:adjective)) for weird person or slow or childish or someone who did a ridiculous thing or dummy or....
e.g. 덜떨어진 녀석. 덜떨어진 여자. 덜떨어진 남자. 덜떨어진 사람...