What Causes Extreme Weakness And Pain In Legs?

My man friend (aged 70) has been having extreme weakness and pain in his legs. He says he can feel it coming on and leaving. He is taking a blood pressure pill and a beta blocker as well as cholesterol medicine. His strength above the waist is normal, but his legs are making him miserable. I know he has had a numbing of his trigeminal nerve because of trigeminal neuralgia about 6 years ago.
What do you think is causing his legs to act in such a way?