Guy may be so enthusiastic (2,4)
I believe the answer is:
on fire
I believe this is a double definition.
'guy may be so' is the first definition.
(as in a guy on a bonfire)
'enthusiastic' is the second definition.
(I've seen this before)
(Other definitions for on fire that I've seen before include "Excited or ablaze" , "Burning" , "in flames" , "Eager" , "Enthusiastic" .)