Callum Livie, 26, used his iPhone torch "app" after his girlfriend Samantha Preedy gave birth alone and in darkness at Edinburgh Royal Infirmary.
She managed to unwrap the umbilical cord from her son's neck before shouting to her boyfriend: "I've dropped him, he's in the pool and you need to find him."
Samantha, 28, gave birth on December 8 during one of the worst winter storms in living memory when a power cut, and the failure of back-up generators, left the birthing pool with no lights.
Callum, from Bonnyrigg, Midlothian, said: "I came in and couldn't see anything. I could just hear Sam saying he's in here somewhere.
"I pulled my phone out and I could see where he was, and shouted to the midwives to grab him. It was a bit of a shock. He might have been under for a minute or two, it all happened so fast.
"Nobody could believe how it happened, but I'm just over the moon that the wee man made it okay.
I'd used the app a few times when I'm trying to find stuff in my van, but I never thought I'd have to use it to help deliver my own son. I really would have been snookered without it.
When the lights failed, the midwife briefly left the room to find out what was happening. Callum said his girlfriend then gave birth alone in the dark.
Midwives then returned to the room in time to pick up newborn Braedan while Callum shone a light into the pool.
Samantha, now a mother-of-five, said: "It's lucky I'm not a first-time mum or it would have been completely different. I've had four other babies and so I knew what to do.
"The midwives were more shocked than I was. I got the cord unwrapped from around his neck, but I lost him in the pool. Let's just say he's really glad he downloaded that torch for his iPhone."
Consort, the firm that runs non-clinical services at the infirmary, is set to be fined around £100,000 after parts of the hospital were left in the dark for 11 minutes.
Well done Callum! What a traumatic experience for the family to go through.