Are you able to say what type of fatigue your dad is experiencing. There is a difference between bone-deep fatigue and physical tireness, and lassitude/sleepiness.
In older people there is a tendency especially in those with neurological conditions to have late afternoon sleepiness, some times called sundowning.
On the other had PD can make you feel very tired, especially if slowness of movement is a main symptom, everything takes so long to do. There are also the medications............. sinemet and agonists can give sleepy phases.
So it is a matter of identifying what is causing the feelings of fatigue, as well as trying to find a way of dealing with them.
Exercising or keeping moving is one way forward, especially if drowsiness is the main problem. If it is not, some supplements may be helpful. I would not think that the so-called energy drinks would be good. They are really for people who are putting out a huge amount of energy and need more easily absorbed calories. I don't think they are good for sedentary healthy people, let alone a person with a neurological condition. Co-enzyme Q10 may help. But ask about the best one to take, some do not get absorbed easily. Look for the posts of people who are taking supplements, and see Ricks post on the study by Blaylock et al. You mention B12, if there is a diagnosed deficiency, or anaemia, this might need to be treated with injections or an easily available type of b12. There are posts about this too, look especially for those by Mrs Doughtfire (not sure I spelled that right) who knows this subject well.
Other than that, a healthy diet with.......... fatigue is a hard one, it is always difficult to identify the cause, but retaining an interest in something creative or absorbing could also be a key, our brains switch into a different mode when we are really engaged in something we like doing, and can override our other more physical feelings.......... it is very important to enjoy doing things, there is a dopa link to enjoyment too.