He needs to keep doing the job to the best of his abilities. And while he does, he needs to keep his eyes and ears open. He should notice, and keep track of some basic issues. Are there any younger employees that are being asked to work these extra hours, do these sorts of tasks? Is this a reasonable expectation based on amount of work the company has, or do they really have a big crew of geezers only that they've suddenly assigned to midnight shovel duties in the desert?
On some jobs there's just a sort of "let up on the older people with seniority" mentality that causes older workers to feel they are deserving of slightly special accommodation, when legally this is not the case. If the job entails everyone at some times having to work these extra hours, do these sorts of assignments, and this has been the nature of the job up to this point, whether or not your father has been being asked to do it before, it is going to be very difficult to prove it is a pattern of systemic discrimination against older workers. And that is what you'd need for an EEOC complaint.
It is quite legal for the company to begin asking everyone to do the same amount or work the same number of hours, even if your father, by virtue of his advancing years, no longer feels able to do it. They are not required to make accommodations for him. And it is quite possible they may be actually encouraging him to quit by increasing the work load. But unless there is a clear cut pattern of them doing this to older workers, turning up the heat on them as they age, then it is probably not going to be determined illegal.
If he is no longer able to do a good job, and his health and strength are failing, then they do need someone else, and he needs another job, or needs to retire. They have no obligation to keep someone who can not do the job. This is certainly an "at will" state.
But he needs to keep doing the job to the best of his abilities as long as he works there. Even if that is not perfect, or is not as fast or as well as it once was. They can ask him to do something, but if it is an impossible task, and he has done his best, that's all he could reasonably have done.
If they elect to fire him in these circumstances, they will be in grave danger of having increased unemployment rates because he will probably be able to draw unemployment insurance for a while. And it is much more likely to give the appearance of possible age discrimination if they give someone an impossible assignment and then fire someone because he's "too old" to do the job. I really sort of doubt they'd do it.
But that's not to say they can't change or increase his work assignments, or that they can't start to criticize his work, or tell him that his performance is slipping so that he may get fired. They may be trying to give him negative feedback, even up to threatening to fire him, in the hope that he will self terminate, in other words, will quit.
Then when he does, and he sees that attorney, he'll be told there is no case, because there's not enough evidence, especially with his deciding himself to quit, that they were discriminating against him because of his age. He would first need to file a complaint with the EEOC, and let them process the complaint and determine if there seems to be a pattern of discrimination at play. It will take a long time, will not get him quickly reinstated or reimbursed.
So your father needs to keep working, doing his best, accepting the extra assignments. If it's not working, if he doesn't allow them to goad him into quitting, they won't be willing to do it forever.
(Incidentally, he's not working unpaid overtime, is he? Is he hourly or salaried?) Ignore the threats, criticism and lack of positive reinforcement. Meantime, he needs to be doing some thinking about how much longer he can continue to do this work, whether he needs to consider retirement or another type job maybe with less stress and hours required.
If he is fired, he will need to file immediately for unemployment insurance. Then he can talk to that attorney, file that EEOC complaint. Even if he were to see one now, he'd be told that nothing illegal has happened yet. And may never happen.
