09/14/2011 at 04:00 PM EDT
Did Bristol Palin
get pregnant just to spite her mother, Sarah Palin?
That's what her ex-boyfriend – and the father of her 2½-year-old son Tripp – alleges in his new book Deer in the Headlights: My Life in Sarah Palin's Crosshairs
, according to a copy obtained by the Associated Press.
In the memoir, due out Sept. 20, Johnston, 21, says Bristol was angry when she discovered Sarah Palin was expecting a baby. Instead, she felt she should be having a baby and, according to Johnston, told him in March 2008, "let's get pregnant."
Among the other revelations in Johnston's book: precisely when the two became sexually active.
Though Bristol wrote her in own book that she got drunk on wine coolers and lost her virginity to Johnston on a camping trip, he says they were sexually active earlier in their relationship. (They began dating in 2006.)
He also writes that he was "too dumb" to use protection when having sex with Bristol.
Despite claims by the Palin family
, Johnston writes that he was present for Tripp's birth
, that Sarah Palin wanted to be called Mommy Sarah (not grandma) and that Todd Palin reacted angrily when he learned Bristol was pregnant at 17, telling Johnston he had to quit school and get a job.
Johnston also writes that Sarah Palin wanted to adopt their son Tripp to "avoid a scandal."