You may have seen tall sitting chairs in the surgeon's office waiting room. That's because it takes a while to be able to sit with the knee at a 90 degree angle. Go to Walmart or Target, somewhere like that, and find a tall "stool" for him to perch on, especially in the kitchen. It will really help. I found one of these for my mom and she loved it. In fact, she still uses it to sit in front of the sink and wash dishes or cut up vegetables. It takes the pressure off of her back.
Buy one of those handheld urinals for him to pee in. It will make it easier for everyone. You may need to find this in a medical equipment store.
Make sure you get the machine that moves his leg FOR him from the hospital. It's great! It helps get range of motion back and prevent blood clots in his legs. It will make his PT go better, too. Go exactly by the directions, and if you have trouble understanding anything, call the rental people the hospital gives you a phone number for. They are wonderful and will talk you through any issues. If the hospital offers for you to rent a machine that has cold water that circulates around his knee, spring for that, too. It's worth it. (Insurance doesn't usually cover it.) However, you need an ice maker in your freezer for that to work because you need plenty of ice at your disposal. A tip for this machine is to have several small commercial ice packs at hand in the freezer as well as ice. You put the ice pack in the water holder. Then you put ice in it, and fill the rest with water. The ice pack will prevent the ice from just melting away in the water. And it will stay colder a lot faster. This will save you a lot of time and running around while you're trying to do everything else. AND it fits very well with being used in tandem with the machine that bends his knee.
You can tell that I've been through this before. My mom had knee replacement surgery and my son had ACL surgery. They didn't have the ice machine back with my mom, and that would have helped so much! I jumped at the chance with my son, and once I got the tip for the ice packs (I kept running out of ice), it was smooth sailing.
Make sure you give him stool softeners - the pain meds will REALLY make him constipated. Don't wait for him to get constipated, either. Prunes in addition are smart, too. You'll be able to gauge it by the consistency of his poops. Sorry if that's TMI...but it's SO miserable to go through major-league constipation after surgery. Prevention goes a long way to helping not have this to contend with, including not having to have an enema done...
Writing down his medication and really managing it is part of the key to him getting through the worst of it after the surgery. Make sure you understand the directions. Make SURE you ask what time he can have his FIRST dose. The stuff they give at the hospital wears off fast. Depending on how long it takes to get him prepared for the trip home, the driving, getting him settled, etc, it will have worn off and he's in misery. And if the pharmacy is CLOSED, you are in for a possible trip to the emergency room. (That happened to a friend of mine. Terrible planning on the part of the hospital personnel, if you ask me.) If you can, send someone else to pick up the medicine while you get him taken care of and prepared for getting home.
DO NOT let him just "soldier it out" with pain. Managing the pain will help him heal. In fact, pain management is so important not only for his knee, but for his brain. Men tend to either be big babies or really stubborn. You need to be the one who calls the shots. Threaten him with Nurse Ratched if he gives you a hard time!
IF the pain meds include acetametaphin, find out (doctor, pharmacist, official website) how many milligrams of it he is allowed everyday. You have to manage the mix of acetametaphin within the medicine. Prescription meds sometimes INCLUDE the acetametaphin along with a controlled substance. What you don't want to do is exceed the acetametaphin limit because it can damage the liver. That means you COUNT the milligrams he's getting every day.
If he gets nauseated taking the medicine, make sure you talk to the doctor quickly. They can give him an anti-nausea medication to take with it.
Write down a daily med/food/liquid diary and keep it current. Make sure he gets plenty of water. (Ask you doctor how many ounces of liquids he should be getting. It's critical that he not become dehydrated.) Set your phone alarm for when he's supposed to get medicine next, and then write down as soon as you give it to him. At first you want to give it as soon as you can (like if the directions are every 4 to 6 hours, give it in 4.) It takes about 20 minutes for the medicine to get into his system to curb the pain. What you DON'T want is for him to run out of medicine in his system and be in agony for 20 minutes because you forgot to give it to him, or waited too long. Make sure you manage his food with the medicine, too. Some meds have to be eaten on a full stomach, but he will have a hard time with a full stomach for a while. Pay close attention. If I were you, I'd research or ask in advance what food you should plan on giving him. Then prepare what you can in advance so that you aren't having to spend too much time in the kitchen. And DEFINITELY shop before you take him to the hospital so that you aren't having to make store runs.
His physical therapy and his willingness to be a compliant participant is very important. It will make all the difference in how his "new knee" will work for him. It is solomnly true that if he doesn't exercise those muscles all around that knee replacement, it can end up breaking. That happened to my father. He wouldn't walk much (yes, he was pretty old) and those muscles were not strong. You don't want this to happen to your dad...
I hope all this helps you. The last thing I want to say is something you might not want to hear. I just lost a dear family member who was in good health, but elderly. It was a real shock. Make sure that he has a will and that you know where it is and that you can get to it. (It shouldn't be in his bank lock-box.) Look at the hospital's paperwork that tells them what to do if something goes very wrong. Really think through this. Ask your father for guidance. He has a right to his opinion and it will make you feel better if you were God forbid, ever have to use it. Make sure that you have him sign a medical power of attorney and a financial power of attorney to whomever should take over for him.
My best of luck to you all. I hope that he bounces back quickly!