I feel for you cj. Parkinsons is different with every patient, so the only advice I can offer you is to spend as much time as possible monitering his care. You know your father, so do not let the nursing homes or hospitals tell you something you know in your heart not to be true. You can read my horror story below as far as institutions go.
I can tell you that most Parkinsons meds work immediately. Unfortunately, nursing homes and hospitals do not realize how important getting these meds on time is. I would also go over his meds and make sure they have not started him on an anti psychotic. In my recent experience, I have learned that nursing homes and hospitals administer these drugs simply as a convenience to them, to make their jobs easier. My mom was lethargic as you describe, and I kept telling them that this is not my mother. They kept telling me to get used to it, that it might be my NEW mother. It took me sitting by her bedside 24/7 for days making sure her meds were administered correctly, on time, and nothing additional added. After a month, she is finally coming home next week.
Keep you chin up, and do not let them push you into something you are not comfortable with.