Stephen Waterson, 26, killed his girlfriend's three-year-old son, Alfie Lamb, in what prosecutors described as a 'fit of childish temper.'
They were in Waterson's Audi convertible in February 2018 coming back from a shopping trip when the incident happened.
Alfie was in the already cramped footwell sat at his mum Adrian Hoare's feet, when Waterson reversed his seat back onto the toddler. With the touch of a button the space can be reduced from 30cm to 9.5cm.
After the seat was moved back, Alfie began crying and the court heard Waterson said: 'I won’t be told what to do by a three-year-old.'
By the time they got home Alfie had stopped breathing and three days later he died in hospital. His death was the first time someone in the UK had died from crush asphyxiation as a result of an electronic car seat, police said.
When questioned, Waterson lied to police about what happened and threatened his girlfriend and two friends who were also in the car.
In sentencing, Judge Mr Justice Kerr told Waterson: 'I do not find you were annoyed with Alfie and moved your seat back because of that annoyance.'
But he said he was satisfied he moved his car seat back twice 'for your own comfort'.
Waterson is the adopted son of former Conservative minister Nigel Waterson who submitted a character reference to the judge saying his son was 'not all bad.'
Alfie's mum Adrian Hoare, 24, was cleared on manslaughter but found guilty of child cruelty in March this year and jailed for two years and nine months.
Speaking outside court, Alfie's grandmother, Janis Templeton-Hoare, slammed the sentence: 'It's not going to bring my beautiful little grandson back. I say a life for a life. I've got no feelings for Stephen whatsoever. He's an evil, nasty person. I hope he rots in prison. I mean that from the bottom of my heart.'