Does he take 1 pill or 2 at time? They do have a short active time frame - 4 hours per dose or so on average- sometimes less than that..

My dad is taking 2 C/L pills, recently upped from 4 times a day to 5 times in 24 hrs as needed..per drs agreement.
Adding other meds at his age (80+) came with too many side effects ( hallucinations, delusions, anxiety etc)
He was able to use Mirepex as an add-on med for short times at low doses in past years.

I think your dad should see/call the doctor and explain how the dosage level is not working for him. Dr may even suggest to up his pills a bit over the phone as a test..
The dr should be very flexible about meds adjusting as it is an ongoing balancing act so communication is important.
If not find another dr..

miralax or a generic is what my dad uses for constipation ,as well as yogurt a couple times a week.