"If the son clearly qualifies as a dependent on my client's return, can my client just decide to not claim him as a dependent thereby enabling his son to claim the $1,200 stimulus tax credit?"

Isn't it your job to file their taxes according to the status that applies, and not what the customer thinks they can leverage to their own interests? Sure, your Client can just decide to do whatever they want, but your professional responsibility is to do what applies per the regulations.

By the way:

You asked once under Lacerte and once under ProSeries. You seem not to be asking a program question, but a Tax discussion. You didn't need to post Twice, if you had posted in the Tax Talk section. It's all one community, so you asked the same volunteers to do twice the work.