Oliver King, 12, collapsed during a sports lesson at King David High School in Liverpool.
He had an unknown but treatable condition called Sudden Arrythmic Death Syndrome (Sads), which kills 12 people each week.
Oliver's family believe having more publicly-accessible defibrillators will help cut the number of people who die from Sads.
For more information visit the Olive King Foundation website here.
More top news
In her message Theresa May praised the "inspirational bravery" and "unbreakable spirit" of Mancunians in the wake of the blast.
ITV News has obtained exclusive footage of the moment armed police raided a house believed to be linked to the Manchester suicide bomber.
Mancunians have come together in defiance after a suicide bomber killed 22 people at a concert.