He’s a short head in front! (3)
I believe the answer is:
Here is my best explanation:
'he's' is the definition.
'a short head in front' is the wordplay.
'head' says to take the initial letters.
'in front' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'short' is 's'.
(Other definitions for son that I've seen before include "Male scion", "Native", "Heir", "Boy", "The kind of child", "A relative", "A child", "Young boy", "Male descendant" and "Male offspring".)