Accused Pankaj had come to the victim's house at Vijay Vihar area in west Delhi in the afternoon to meet his father.
Finding the boy alone, he sexually assaulted him, police said.
After the incident, a neighbour heard the boy crying and informed his father.
The accused was arrested after medical examination confirmed sexual assault, police said.
Kareena Kapoor Khan snapped with baby Taimur at mom Babita's house
Photos: Amruta Fadnavis at a fashion and lifestyle exhibition
Photos: Sushmita Sen stuns in a black dress with thigh-high slit
Photos: Bhumi Pednekar, Saiyami Kher at a film screening in Mumbai
Dreaded dons! Meet Bollywood's gangster brigade