Donna O'Halloran said David had drunk half a bottle of vodka in less than an hour before going clubbing with friends in Stirling, and urged other students to take more care when heading out.
Police are still searching for the 18-year-old, who studies at Stirling University, more than two weeks after he went missing on Friday, January 18, after visiting the city's Dusk nightclub.
Ms O'Halloran fought back tears as she recalled her parting words to David the day before he vanished. She said: "I said 'I love you'. Those were always the last words I would say to him. 'I love you, son'."
"And he would go 'I love you'. It doesn't matter if he's in a crowd or a party, he will always say 'I love you mum'. He didn't care."
Ms O'Halloran said Dusk's bouncers ejected her son because he was drunk, and criticised them for not alerting the university to his state. She said: "He was getting escorted out of that club for having had enough to drink. His friend stepped in and said 'I'll put him in a taxi'. If his friend hadn't stepped in, David would have been put out."
"These bouncers need to learn something. This has to be stopped. These young ones need to be protected."
Earlier this week, CCTV emerged showing David crossing Henderson Street in Bridge of Allan at 2.19am – more than a mile away from his halls of residence, where he had been seen half-an-hour earlier.
Ms O'Halloran, of Kilmaurs, East Ayrshire, fears David may have caught hypothermia as he wandered in the cold.
A picture is emerging of David's movements the night he went missing. He was seen getting into a cab near Dusk in Stirling city centre at around 12.30am, was spotted walking along Airthry Road from Causewayhead at about 1.15am and at the roundabout outside Stirling University at 1.57am.
Staff at Graham's Dairies reportedly saw David and pointed him in the right direction for the university, but it is understood he walked off in the opposite direction.
Police confirmed the company had been helping with the investigation by providing witness statements.
David is 5ft 6in tall, of slim build and has brown hair with blond highlights. On the night he went missing he was wearing a white T-shirt with a black and green checked shirt on top, black slim-fit jeans and black trainers. Anyone with information is urged to contact Central Scotland Police on 01786 456000.
The nightclub did not return calls from The Herald.
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