Ga Resident with Alabama nonresident income

Helping my son with his taxes and discovered that his employer (a Georgia company) allocated 97% of his 2021 compensation to AL and paid his withholding to AL.  But he is a full time Georgia resident (owns home in GA) and last year commuted to AL to work on a project there.   His employer only allocated one pay period to GA.

We finished his Federal taxes, Ga State taxes and an Alabama nonresident income return.  While GA gave him a credit for the taxes paid in AL, he still owes quite a bit to GA.  I presume this is because of under-withholding for Georgia.  (he did not notice during the year that his company was taking out AL state taxes.)

When he questioned this with the HR office, the individual said the company "might" be able to issue corrected 2021 W-2 forms and re-allocate all his compensation to Georgia, but she was not sure...  And he would need to file extensions for all the returns while she sees if they could do this.  Bite the bullet and pay Georgia, or see if his company can fix this mess they created?    

Is there a way to do a "what if" Georgia return to see if he is still going to owe Georgia $$ even if his compensation is all allocated to Georgia?   Thanks for your thoughts.