It is hard to say without seeing what is happening.
If it is foot drop, he could have an AFO fitted and still be mobile for a while.
Chris never had foot drop in the typical sense. Instead he began to have coordination issues, and the muscles in his hips wasted to nothing, then his upper legs began to waste and the coordination became so noticeable. He used to bounce of the walls in the hallway, literally and the things hanging on the walls were always skewed. He would laugh and say he tried to bump in the same places on the way back to straighten them up again.
The spasticity that developed in his legs was the most dangerous part as he could not will his legs to move correctly.
This, for Chris, took at least 8 months to go from walking but having the first minor falls, to only being able to stand for transfers with a lot of effort and assistance from me.
Your dad needs to be checked for foot drop, and have his walking ability checked by an OT to determine what is going on.
