A man tried to drown his 6-week-old son in a flushing toilet during an armed domestic dispute that led to a SWAT standoff, Pittsburgh police said Tuesday.Dennis Gateley, 31, was in custody awaiting arraignment on charges including aggravated assault against the infant, endangering the child’s welfare and illegally possessing a firearm. He was also charged with assaulting and threatening to kill others in the house. Gateley’s brother told police that he found Gateley, the child’s mother, and the child’s paternal grandmother arguing in a bathroom late Monday. The grandmother had told police she saw Gateley punching and pushing the child’s mother so she grabbed the infant and ran with the child’s mother into a bathroom to get away. Gateley forced his way into the bathroom and his brother saw the women “yelling at Dennis who was holding the infant child upside-down pushing his head into the toilet,” police said in the criminal complaint.