Sorry to hear about your dad mate, but it's beautiful you think of him so highly
My dad's had Parkinsons for a long while, as have a few others I know, I've been following the research and their progress for a while. Unfortunately none of the people I know with Parkinsons are interested in support outside their direct relationship with their GPs ( who mostly don't know much about Parkinsons )
But swimming's one of the best things your dad can do, if he's having trouble can you get him to wear a flotation device? I know there can be a bit of pride about wearing one in a public place... but swimming's so good for stages of PD. In Australia I've seen water exercise used as a recognised form of therapy for PD and it did work for my dad as a part of slowing his Parkinsons progression
No, you seem awesome and loving. I wondered about those compounds too back when Dad was diagnosed. I think you're right about potential deficits after use. Am not a physician and there are people here who are better qualified than I to explain the mechanisms
There are some biological compounds I'd look at first before directly going for the pure stimulants
Mucuna pruriens seed could be a good place to start if he's not already on meds ( buy it in, or only grow the hairless subtype )
CBD is approved in Australia for treatment of some PD symptoms. Not sure about THC, but CBD is definitely on the list and I've heard ( and only heard- not seen yet ) good things of it for this. Not sure about legality/ accessibility where you live. Is most likely to be well tolerated alongside other medications
Ibogaine root bark has been reported to reduce symptoms of PD too. There's a bunch of glowing reports on that if you google ( I do not trust glowing reports on Google much, but low dose ibo is awesome and I'd personally give it a go if I ever got a diagnosis) - but if you're recommending it to anyone else- particularly if they're already on other medication- I'd take real care to find a sympathetic primary health care professional and a good clean source of ibogaine root bark
Was most interested to hear psilocybin/ mushrooms mentioned above, now I'll have to do some reading :)
All the best for your and your family's journey.