Please explain the parts in red.
'How come you feel over, son?' said Uncle Vernon, in an unnaturally quiet voice , the kind of voice he might adopt at a bedside of a very ill person.'
'T-tripped,' said Dudley shakily. "And then -'
He gestured at his massive chest. Harry understood.
(1) 'Fell', not 'feel' -- he fell down.
(2) 'T-tripped' -- is a written transcription of a stuttered statement. 'I'm r-r-really af-fraid of g-g-ghosts!'
(3) He pointed to his own big chest. (I don't know why from the context supplied).