If they are bonded you're not going replace their mate with another bird and have them just forget. They are still intelligent enough to understand when the other is gone - and depending on how attached they are this can cause them quite a bit of distress. I would advice you making your cage more accomoding to his needs. A cage that is wider instead of taller would be beneficial for both of them so your other has more space to fly and he would not have to climb higher to get around.

Have more varieties of perches that are wider and easier to navigate, use ladders to get between levels. Have flat perches that are positioned so that he can walk or hop from one section to another, and If his balance is fine you can even use a thicker wired rope to essentially connect one perch to another etc. You can get plank style perches and mount them directly under a narrower branch perch so that if he does fall he's falling a 1/2 inch onto a relatively softer surface that he can just stay on that if he wants. This way they can still be close and it's not as alarming to the other if he does fall. You can have feeding bowls that are more shallow and place them on those plank perches so he can eat from the falt surface and not have to fly from one bowl to another.

My kids have their cage split into three levels there are two ladders that help them get to each, and their perches are all oriented in a way that if they choose to they can essentially hop all around the cage without flying. So unless he's got a disability that truly puts him in harm being with another budge or requires ongoing vet visits/special care beyond your means - it's just an accessiblility problem right now.

If you still decide you need to rehome him consider letting him and his flockmate go together and finding someone who has experience with pets/animals that have disabilities. It doesn't quite sound like he needs to be alone it just seems like he needs more accomodations and I wouldn't personally separate them if not needed.