Excited seeing son's birth (7)
I believe the answer is:
genesis
'birth' is the definition.
(thesaurus)
'excited seeing son's' is the wordplay.
'excited' indicates anagramming the letters.
'seeing' anagrammed gives 'genesi'.
'genesi'+'s'='GENESIS'
(Other definitions for genesis that I've seen before include "the start of something" , "First book of the Old Testament" , "Origin (of the Old Testament?)" , "Origin; Bible book" , "Origin or formation of something" .)