It's wonderful that you are so interested in your Grandfather's health and in understanding his Parkinson's disease. I realize my response may be too late since you were visiting him a month ago.
Many people with Parkinson's take medication several times a day and many take Sinemet. Sinemet is probably the medication that works the best for PD (there are pros and cons to every medication - age, progression of the disease, response to other meds are a few factors to consider). So ... Sinemet is commonly prescribed. I'm not familiar with the other medication.
Your Grandfather's 'confusion' may be a PD symptom or may be related to age. It is true that many people with PD have cognitive issues (basically the 'thinking' process). Occasionally it can be a side effect of meds - I have never taken Sinemet and am not familiar with the side effects.
PD is a movement disorder ... hence, it often causes slowness or stiffness. Medication helps to improve movement, but the issue will always be there and willl worsen over time. PD is a progressive disease.
Exercise! So glad you asked. I am an avid believer in the benefits of exercise - any exercise - for people with PD. The more rigorous, the better. I have been involved in a free (hence I am not advertising) boxing-based fitness program for people with Parkinson's. This program is unique to this group - not a franchise, etc. To learn more about the program, go to www.youtube.org and search on 'Rock Steady Boxing'. I would encourage your Grandfather to get involved in some type of exercise program. I feel that exercise has slowed my progression and allowed me to maintain a lower level of medication (hence reducing side effects).
Best of luck to you and keep on giving your Grandfather (and Grandmother or other caregiver ... because they are angels) your support.