Considering that he has recently lost his job & getting irritable since a month then he might be suffering from depression, in elderly depression, agitation, irritability, forgetfulness is common.
But before we label it as depression, considering the age of 75 yrs & forgetfulness, he might need brain imaging like CT scan or MRI to rule out any brain related cause.
Next Steps
Visit a physician who will assess him, order investigations & if they are normal, then will refer to a psychiatrist.