My mother is having a similar problem. She is 88 and had a stroke in July 2012 and had PT through the end of last year and did very well. Then in February this year her left leg started to get weak again. She took a month of PT which helped some and then she had shoulder replacement surgery and both legs, but primarily the left leg, got extremely weak causing her to have to stay in a rehab facility for 6 weeks. No trouble from shoulder replacement just her legs. They have improved to the point that she can walk unassisted but she trudges. She says her left leg feels very heavy and she has a lot of trouble getting out of the car. Her leg just seems not to get the message from her brain to move. I just read an article about an electrical stimulation to the muscles in the leg and wonder if that would just be a short term thing or whether when they get the muscle working again if it continues to get better.
Hi
Sorry to hear about your dad, but it's great that he is up & about.
As his left leg is weak it will benefit from some targeted strengthening exercises. These are a few I found useful.
Lifting the leg high while walking, may need support. This exercises the quads which help to pick the leg up when walking. Can also be done on machine targeting the area if the leg is strong enough.
Sideways walking, also may need support, keeping the legs straight. I found this helped to turn my foot around from the side to the front.
When he is a bit stronger, he then needs to consciously try to walk as well as he can, to try bringing his leg straight through, toes pointing forward. The more he practices doing it right, the easier it will become.
Basically, he has to keep working on it to maintain progress & gain more. Also remenber, stability is more important than speed. Speed will come when he is stronger & feels safer.
HTH
Sue