Simon Davey, 33, spent a year posing as a friend of Michael and Claire O'Brien only to swindle them out of hundreds of pounds.
The couple, from Wales, founded the Dylan O'Brien Foundation following the death in June 2012 of their four-month-old son from bronchial pneumonia and an undiagnosed metabolic condition.
Davey befriended the Welsh couple via Facebook. He took a £250 fee plus £49 for a banner, promising them publicity at a black-tie corporate sponsor event in London.
The couple then contacted the police because they had not heard from Davey and were concerned for his welfare.
But they were informed that he had become an inmate at HMP Birmingham and that they had not been booked into the corporate event.
Mrs O'Brien said Davey's actions had left the couple feeling "sick and stupid" – and he was this week jailed at a sentencing hearing at Cambridge Crown Court.
Mr Benedict Peers, prosecuting, said Davey also conned cash out of Nicky Vere-Compton, of the UK Pie Party, which raises cash for children's hospices in East Anglia by charging people to pelt her with custard pies.
He said Davey stole £375 from Ms Vere-Compton after saying he could help her set up a fundraiser in America.
The prosecutor told the court that at the end of 2014, Davey also defrauded Desired Construction Ltd, by whom he was employed at the time.
Davey pocketed £790 and then claimed to have bowel cancer when asked to repay the money, Mr Peers said.
Davey pleaded guilty to two counts of fraud by false representation, one count of fraud by abuse of position and a further count of theft. At an earlier court case he gave his address as Arthur Road, Tipton.
Mr James Dignan, for Davey, admitted the defendant's actions were "despicable" but said they were prompted by the fact Davey was in debt and "impulsive". But Judge Gareth Hawkesworth, jailing him for two years in total, said the impact of Davey's crimes, as well as their number and the type of people he targeted, made his offences more serious.
He said: "These are not impulsive frauds. They are thoughtfully planned and executed and involved targeting people you feel able to con and manipulate.
Davey served an eight-month jail term several years ago for defrauding his employer out of £17,000.
Mr O'Brien said: "He almost destroyed the launch of our charity. If it wasn't for the generosity and donations from the public we would have gone under."