Hello and thank you for asking your question. My name isXXXXX will try to help.
Slipping on hard wood floors, and abnormal stance, is a sign of arthritis in the hips or lower spine. It is a common problem as they get older. Feel down the outside of his thighs and see if you feel a loss of muscle that used to be there. That is a sign of hip arthritis. It is because they have not been using their thigh muscles, the way they used to, so they lose muscle mass. The only way to confirm the diagnosis, of arthritis, in the hips and spine, is to take x-rays. You can try treating him symptomatically.
If he does not have any pre-existing health problems, and he is not on any medication, you can try giving him aspirin. The dose is 5mg per pound, twice a day, with food. If it helps, it is probably arthritis. If he is going to need aspirin, or another pain medication, long term he will need to have blood work done. This is because the pain medications can affect the liver, kidney, and lining of the gastrointestinal tract. So you want to get baseline blood levels. Then when you recheck his blood work, in 6 months, you can see if the pain medication is having adverse effects.
You can also give him nutraceuticals medication to help his joints. These are not pain killers but they help put fluid in the joints. You can think of it as putting oil in your car so the parts do not grind. The more joint fluid he has, the less likely his bone cartilage will grind. You want to get one with glucosamine and chondroitin. One of the best ones, because they have studies to back it up, is "cosequin." You can buy it online or at larger pet stores. You do not need a prescription.
It also helps to put area rugs down on the hard wood floors. This will help him get some traction.
I hope this helps.