boy,
are you against taking any medications? how about aspirin?
it's hard to believe you asked that question. it's similar to asking a diabetic if insulin might help.
bottom line, he can try l-dopa for a few weeks and stop it if he doesn't like it, HAPPENS ALL THE TIME, l-dopa response is used as a diagnostic tool. it's a natural occurring amino acid plus another compound that suppresses an enzyme which changes l-dopa so it can't get into the brain. eating a candy bar is likely more harmful. this is not a powerful cancer drug, not a psychotrophic drug.
i almost think it should OTC, well not really.

i suggest you take the opposite view and be thankful there are medications to treat his pd symptoms rather than think of them as necessary evils. if it was my father i'd be demanding he start on meds. and then learning all i can about how to help him make the meds work as well as possible and other things you can do to maximize the quality of his life. that might mean getting him into counseling, into an exercise program, taking an antidepressant. i'm not a healthcare professional but the best approach to treating pd is a team effort, MD, neuro, mental health, physical therrapist, family members. if you in 4 years are still asking if meds can help you imho shouldn't be involved in helping your dad if other family members are available or you need to find a social worker to guide you. being brutally honest.
i'm 59, when medicated, you wouldn't notice i have pd, when my meds wear off, i'm 90 mentally and physically. that's how dramatic the benefit of l-dopa can be.

keep in mind that 1 egg has grams of amino acids that are precursors to dopamine yet the avg daily dose of l-dopa is ariound 500mg, at least 6x less than in 1 egg and most is broken down and never gets into the brain.

i didn't go back thru all your posts to see if he has already tried l-dopa.