Answer: He should add a single dot below the symbol to correct the electron dot diagram of an aluminium atom.
Explanation: Electron dot structures are the structures in which valence electrons around an atom are shown. The electrons are represented as dots in these structures.
Aluminium is the 13th element of the periodic table.
Electronic configuration of Aluminium is given by:

Valence electrons of aluminium = 3
In the given image, 2 valence electrons are shown and total number of valence electrons are 3. So, in order to correct the diagram, he should add a single electron below the symbol.