Unfortunately, what was formerly called "PMA" is increasingly recognized as progressing similar to "ALS" with two exceptions that go slower -- flail arm or flail leg variants. You might ask the doc if he has either of those, or check out the descriptions on line. Apart from those, pts with only lower motor neuron signs at diagnosis (the classical criterion for PMA) gradually add UMN signs and end up about the same place the same time. Sorry to be a downer. If your dad doesn't have FA or FL, your doc doesn't keep up with the literature and shouldn't have told you that and you might want to get a better doc.
The main reason for telling you this is that this thread (PMA) doesn't have much for that reason (PMA is not really used that much these days because it doesn't say anything meaningful, though FL and FA do), so I would check out the CALS forum under the regular ALS forums for yourself, and the PALS forum for your dad, including the "newly diagnosed" thread.
Welcome to the community.