Yes, the testicles are exquisitely sensitive to touch and there is a huge release of adrenalin when there is excessive force applied to these organs.
Testicular pain is referred to the lower abdomen, mesenteric plexus, and causes men to stop abruptly what they are doing, lie on the ground, close their eyes and bend their knees.
A heart attack could certainly result from severe testicular pain from squeezing.
According to the Irish Mirror, a source close of the family says that it wasn't a savage attack, just a joke, and that it wasn't malicious:
A school teacher and one of the youths performed CPR at the scene before the emergency services came which could have saved the teen's life.