Midnight, since I don't know you or your parents please don't take any offense with another perspective I'm going to throw out regarding your Dad's situation.
I wholeheartly agree that if your Dad is in pain and not moving around well at this point after his surgery, you should immediately take action and talk to his surgeon. After all, your father's well being and health are at stake! But I think how you begin that discussion is very important.
Unless you were present at one or more of the previous meetings with the doctor, you don't really know what was said by either the doctor or your Dad. Because of that, I would not begin the conversation on an angry or accusing note. To do so would immediately put the doctor in a defensive position and that would not serve your Dad well. A better approach might be to tell the surgeon that you have some concerns about your Dad's progress in recovery and would like to have some questions answered about the treatment following his surgery. If the doctor is a good one, he/she will talk openly with you and you will have maintained a good relationship with him/her by starting the conversation in this way. You always have the opportunity to get tougher if the doctor cops an attitude, doesn't provide adequate answers, or refuses to help your Dad. And you SHOULD get tougher if any of these things happen - up to and including legal action, in my opinion! But like I said, if you weren't there, you don't really know what your Dad might have said regarding therapy. Some people don't want it or don't understand the surgery and think they don't need it.
Be prepared before you try to discuss your Dad's case with the surgeon that today's laws regarding medical information require that your father must agree in writing for his doctor to discuss particulars of the case with you. This would be easy if you had a physical meeting with the surgeon and your Dad was present. But if you are planning to talk on the phone, your Dad will need to sign a release form and you'll have to take it or fax it to the doctor's office before you talk.
Another big factor in your discussion with the surgeon which was not mentioned in your original post is how active your Dad was prior to surgery. If he was active, up and doing things but having pain because of the need for surgery, what is going on now is a major life change and definitely not right. But if he was used to sitting in a chair most of the day before surgery, it could take some effort to change that habit. You didn't really say whether he wants to be more active or not. Therapy, including stretching and pushing to get normal range of motion for your new knee, is very difficult following this surgery and a person has to make a commitment to that effort. Plain and simple, it hurts for the first couple of weeks!
I have a 72-year-old friend who was not willing to make that commitment and didn't want to endure the pain of therapy on his first knee replacement. He refused to do what the therapist asked in terms of exercises and because of that was "released" from therapy at that point because he was not making any progress. The result is that his knee does not have normal range of motion and he has somewhat of a limp because of it. On his second knee replacement, he was a much better patient, did therapy, endured the pain and that knee works great. His doctor recommended that he go back in on the first knee and have a manipulation done to break up the scar tissue that formed during his inactivity, but he is still choosing not to do it because of the fear of more pain and therapy.
I hope what I have written is of some help to you. As I mentioned at the beginning of this post, I did not mean to offend you in any way, but only to offer some things to consider. I commend you on your caring concern for your father. He needs your support to get past this problem and begin to get an active life back. Please write more to let us know how things progress in the coming weeks. I pray that you can get some answers and get him on the road to a more active and pain-free life. He's too young to be relegated to a chair!