Be beside oneself (with): "grief" , "glee". To be in a state of very great, uncontrolled emotion. ( in grief). In a state of extreme excitement or agitation. "They were beside themselves with glee".

Hi Safaa see this link .http://www.randomhouse.com/wotd/index.pperl?date=19961114

It is an idiom. He felt a lot of grief. This is an expression not a literal meaning.

'Beside yourself with something' is an idiom meaning 'in an extreme state of emotion or unable to control yourself because of the strength of emotion you are feeling'. = The father was overcome with grief. He was so sad that he could not control his feeling. 'I was beside myself with joy.' 'Sarah could not speak. She was beside herself with anger.' 'I laughed so hard I was beside myself.'

It means he was so too sad to control himself when he lost his son