Thank you for your additional information.
Yes that does make sense. It is very possible that he has an injury to the spine Often a rupture or herniation of a disc in the neck will effect the front legs.This can happen from something as simple as jumping off the counter, slipping on the floor.
IVDD is a common cause of disc injury in cats.
Here is additional information.
There are other underlining health issues common in older cats that will cause weakness in the legs, however, they most commonly effect the hind legs. These possibilities would include Diabetes, kidney Disease and Hyperthyroidism.
I would expect to see some signs of illness if any of these were the problem.
I would suggest having him seen by your vet. X-rays may show compression in a disc, however, often a rupture will no show so your vet may recommend consulting with a veterinary neurologist.
Until you can have him seen try to keep him as quiet and still as possible confining him to a small area would be best. No stairs, no jumping until he sees the vet.
I hope all goes well.