Malay dailies reported that Mohd Fitri Wazir, 21, was climbing a fruit tree in a friend's garden with his friend Mohd Mahathir Abdul Aziz, 33, when the incident took place in Kuala Pilah, Negri Sembilan.
According to Mahathir, he heard an explosion, then saw Fitri holding his head in pain.
The two got down from the tree and they encountered an elderly man holding a shotgun.
Mahathir said the man had mistaken them for monkeys.
Mohd Fitri was brought to Tunku Ja'afar Hospital for treatment. His condition was reported to be stable.
OCPD Supt Jaafar Baba said police arrested the man in his 70s and had confiscated the shotgun.
The case is being investigated under Section 37 of the Arms Act 1960 for causing injury with a firearm.