Police Monday arrested Roderick D. Tademy, 36, of Joliet outside his apartment where the incident occurred and questioned him at the police station, the Joliet Herald-News reported.
"The 36-year-old mother told officers her husband had shot the boy," Police Chief Fred Hayes said.
The mother took the gun from Tademy and gave it to investigators, Hayes said.
The child suffered a gunshot wound to the head, the Herald-News said.
"This investigation is ongoing but it appears this shooting was intentional -- not accidental," Hayes said.
Two 6-year-old siblings also in the apartment during the incident and reported to be at the hospital with their mother will be given victim-sensitive interviews, Hayes said.
Police said they planned to search the home after obtaining a warrant.