My mom has a different issue (both knees were replaced) but sounds similar in that she needs a wheel chair for the majority of the parks but can walk some.
As the previous poster said above most lines can let you in with the wheel chair. However some things that may be good to know (I'm assuming your relatively new to disney with this, sorry if you already knew alot of it)
1) Many rides he will have to transfer for, you can't put the wheelchair on the ride. Since he can walk some this is probably ok though.
2) Some of those rides you have to transfer for have a moving platform you have to walk on. Think kind of like the ones at the airport. My mom has some balance issues and has some trouble with these. At some rides they can slow it down if you ask (spaceship earth) at some rides they can't do it though. I suggest asking if this is an issue for him. If they can't slow it I would suggest having two able bodied adults on either side of him to help with balance and get him in his car. Put him in first and one gets in with him and the other in the next car (a healthy adult has time to help one person in and then get in themselves). This way the adult riding with him will get out first to help him out of the car until the second is there to help with walking. The hardest part of these for balance is getting from the still walkway to the moving part and then from the moving part to the car. That is where people tend to fall if they are going to.
3) AT some rides where you don't have to transfer its still helpful to let them know if you CAN This will cut down on lines. For example at the AK safari ride there are only a few vehicles that go to the handicap area. Those that do I think only have one wheelchair slot. However you start and end at the same platform here so you can wheel him almost to the car have him walk a few steps and sit in the car and then put the chair to the side. This way if there is already someone in the wheel chair spot you don't have to wait for the next turn (or force someone that can't transfer to do so). Jungle cruise is like this as well.
4) Depending on how far he can walk some rides may be faster to not bring the chair in the line at all. Example, my mom now can walk a decent distance as long as its not fast and can walk stairs, again she is just really slow. So doing toy story mania line is ok... its a line its not going to move too fast anyway ( a few people will pass us before we get to the actual start of the line but that doesn't bother us) You may get someone behind you annoyed on the stairs that your talking to long and making a gap before the next group but frankly I never care... they aren't going to get on the line any faster anyway. However only do this if your sure he can walk enough as you don't want to be stuck in the middle of the line when he needs his chair.
5) Be patient. Many rides will take longer with the wheelchair, some you will get in faster but I think in the end it still ends up taking longer for most things. For another example some rides have a limit to how many people in wheelchairs can be on a ride at once. I think this is so that if they have to evac the ride there aren't too many people that can't walk off the ride on their own. This accounts even for those that transfer (if you went through the line in a chair it counts) So this may slow you down even in places that have a seperate wheelchair line.
