Was it a dream or was he really whizzed into the future?

Nick Hornby's novel Slam centers around a teenage boy named Sam and the many connections in his life to teenage pregnancy. 
On his sixteenth birthday, Sam's girlfriend Alicia tells him she might be pregnant, and he does his best to avoid her. After celebrating his birthday with his fighting parents, Sam goes to his room and falls asleep. When he wakes up, it is somehow months later. Sam is at Alicia's house, and Alicia has had a baby. 
He lives a day in this future-life, but the next day he wakes up and is back in his normal timeline. Later on in the book, there are two more instances where Sam is "whizzed" to the future and then brought back. 
Throughout the book, there are not any clues that definitively tell us whether Sam is dreaming or actually being "whizzed" to the future, so it is up to the reader to decide which way they think it is. Alternatively, you could just say it is ambiguous and leave it at that.