What if there is a paycheck that he needs

to pay the mortgage and feed the family? How then would he have handled the situation?

The boss's dissatisfaction happened way before the outcome of the experiment. He has been observing the way your son conducted himself in the lab for some time and found it wanting.

Your son needs to be more self-aware. He can tell himself "I don't care what the boss thinks since mom and dad are footing the bills." But at least he should know what the boss thinks before he decides he doesn't need to care about it.  Being oblivious of the effects of what you say or do having on others is a mark of immaturity.

Your son cannot control what his boss say or do. But he should control what he himself say or do, or at a minimum, know what he needs to say and do, even if he is not doing it, or doesn't feel like doing it since the consequence of not doing it is not dire enough, yet.