"He had been gassy for weeks and then he started throwing up," Aiken, 29, tells PEOPLE in its latest issue. "Jaymes [Foster, the baby's mother] and I were trying to be calm, rational parents, so we didn't call the doctors too much, but it just got to a point where he was vomiting everything. He was starting to get dehydrated."
Parker was soon admitted to a nearby hospital and diagnosed with pyloric stenosis, a condition that prompted doctors to decide he needed surgery the following day.
It was in the early hours of the morning, shortly before Parker was scheduled for the surgery at a nearby hospital, that Aiken, who was at his home in North Carolina, "got really panicky," he tells PEOPLE.
So the singer headed to the hospital around 2 a.m. – after second-guessing his impending visit.
Wanted to Know Everything"I was a little worried, like 'I don't think they're going to let me in the hospital at 2 or3 in the morning.' Then I thought, 'Wait a second, I'm the father! Of course they'll let me in," Aiken says. "It was the first moment when I thought, 'I'm actually this child's father!' "
Aiken came armed with a series of questions for the doctors. "I don't like to nag people, but I asked every question I could think of, and I thought of some more and called them back. I wanted to know everything," he says.
"I was like, wait a second, this is my responsibility, I need to take care of this child, and he can't ask questions for himself. It was a reality check, like this is the real deal."
As for Parker now, a beaming Aiken says, "He's fine, healthy and doing well!"
For the complete story on Aiken's new role as father, pick up the latest issue of PEOPLE, on newsstands now