How do I get my Dad to change his diapers when needed? I can smell when they are soiled. He insists "I put clean clothes on this morning".

Does your dad have dementia as a result if his Parkinson's? If so,
he may not smell those odors. The smell of feces, urine, natural gas, chemical smells from household products and other smells may not register with him.

Please don't use the word “diapers” in his presence, it's demeaning to an adult. Instead use words that you would use in referring to your own undergarments... underwear, skivies, briefs, for instance.

Acknowledge that he did indeed put clean clothes on this morning.
Prompt him to go with you to the bathroom. With some clients who don't think this is necessary, explaining it's terribly important to check their skin and avoid pressure sores proves a useful excuse!
Assist him to change his diaper.

If possible, help him figure out a bathroom schedule timed to avoid soiling. Is his bowel habit reasonably regular?

Your profile says your father is 97 years old. I think you need to adjust your expectations of what is realistic in this situation. If you are struggling to deal with his messes, then hire someone to come in to deal with it. But I don't think it's fair to him to think he can help himself anymore. He may also be in cognitive decline aside from his Parkinsons. I realize this is hard to come to grips with, so I don't say it lightly. I'm sure you're already doing a lot for him. Do you have a shower set-up that makes it easy to clean him? A hand-held shower head and shower chair, grab bars, etc?