Police have arrested a 41-year-old man for allegedly beating his 19-month-old son in Tsuen Wan early yesterday. He had been arguing with his girlfriend, the boy's mother, over money matters in the Kam Fung Garden unit on Castle Peak Road when the incident happened, police said. The 19-year-old mother called police after the man allegedly hit the boy's thigh. Officers arrested the man on suspicion of child abuse and sent the toddler to Yan Chai Hospital for a check-up. Police also alerted the Social Welfare Department to follow up the case. Last night, the man was being detained at Tsuen Wan police station for questioning and had not been charged.