Speaking four weeks after his son's disappearance, Alan McInnes was nearly overcome with emotion as he described his "happy, intelligent, outgoing boy" and dismissed those who have given up hope.
He said: "Too many people have said he is no longer with us.
"We have to remain focused he is alive and for whatever reason he cannot come home.
"We are looking for a person and not a body. We will continue to believe this until we are told otherwise by the police."
In a direct plea he added: "David, son, if you can see or hear this, please know we miss you, we love you and we all need you home. Our family is not complete without you."
David, an 18-year-old mathematics and education student at Stirling University, went missing in the early hours of Friday, January 18, after going out with friends at the city's Dusk nightclub.
He was dropped at his campus by a taxi at around 1am, but was later spotted jogging further away from his student accommodation.
The last confirmed sighting on CCTV showed David crossing Henderson Street in Bridge of Allan at 2.19am.
Police have searched more than 5000 houses in the area and drafted in helicopters, mountain rescue teams and trained dogs to check the countryside despite challenging wintry conditions.
Mr McInnes, speaking at a police press conference for the first time, refused to blame the nightclub or the taxi company for David's disappearance and instead urged the public to continue to come forward with information.
He said: "We are not going to blame anybody and we believe the nightclub stewards and the taxi drivers did everything they would have normally done.
"They are not to blame for David's disappearance.
"This is totally out of character for David to go missing. The family consists of four children and we need them all to be complete.
"If you have seen David please, please let the police know. We need to know where he is."
He said David's siblings – half-brother Kaya, 11, half-sister Giana, 4, and step-sister Allanah, 19 – were distraught and just wanted him home.
Asked why he decided to talk publicly now, Mr McInnes said: "There is so much negativity, especially on the social media sites, that people are just writing David off.
"I feel now it is a good time, four weeks down the line, to try to get it out there to everybody that we are not looking for a body, we are looking for David."
Mr McInnes also defended David's actions that night – he had played drinking games with friends.
He said: "A million other kids have done that. Ask any other student on the campus and I bet they do it twice a week."
Police have released a new photo of David with his family and are focusing their inquiries on the crucial period between 2.19am and 3am.
Superintendent Gordon Dawson said: "We are still appealing for the driver of what we think is a white Skoda that was in Bridge of Allan about two minutes before we see David on CCTV. That driver is possibly the last person to see David on the road."
David is 5ft 5in, 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.