How can you help a 90-year-old who has advancing loss of leg strength and wakes in the morning not being able to move his legs?

LT gave a good answer. Please check with the doctor. It sounds as if some physical therapy may be needed. There are some therapists who go into homes, though you'd have to check your locality. Even if you have to take him out, you may be able to learn the technique so you can do it yourself. Good luck,
Carol

Probably you will need to passively and gently move the legs yourself for them to get the energy into the muscles and the fluids in them moving. And if you can encourage some very gentle simple exercises during the day to hold some strength. It depends on the condition. You need to talk to the doctor and perhaps get referred to a rehab person or physiotherapist who can get a prescription for the doctor for exercises and movements that can be done.

Have you seen that little device that you place under the ankles to gently rock the legs and increase circulation? I have always wondered what that might do. Like - 15 minutes before rising. Good old Internet search - if interested look for it.

Well, I have seen nurses take the lower legs, and gently lift them, up and down, each one at a time, whatever is tolerable for them. If you can get them to walk during the day, even just a bit around the house, this would be good for them. Perhaps, if possible, using little ankly weights. I think also massaging the calves with something warming. Is that they are stiff? Or, is it that their brain can't make their legs work? If it is the second, I would definately see a doctor. Also, I would use a pencil, or something you have handy to touch the bottom of their feet, up and down type motions, see if they can feel it. Getting old sucks!!

Take him to the doctor/specialist for an exam and tests to find out what the diagnosis is. The doctor may order physical therapy to strengthen muscles if possible. The therapist will then advise on specific exercises or movements to help with any mobility that's left. This could be a neurologic problem.