Have you tried the following to activate knee extension:
1) patient sitting with feet above floor, no surface contact. PT about 2 m in front, roll a ball (beach ball to start with, very light) towards the weak leg and tell patient to kick. If no reaction first time - do it again, at least 20 times before you give up. If reaction - change into heavier ball, Wikipedia reference-linkBobath-ball, which makes it obvious to the patient (and his brain) that more force is required. Many repetitions.

2) use a vibration plate (cheap ones have not been proved to be less good than expensive ones). Begin in a sitting position and put the affected foot on the plate. Tell the patient not to allow the foot to slide off the plate during vibration, (eg 30 Hz/30 secs). Do 5 repetitions.

3) standing with the affected leg on the vibration plate. You need to stand the non-affected leg on a low stool or similar of the same height as the v-plate to make it possible to do weightbearing over the affected leg. Use same dosage as above.

4) stand the patient on two bathroom scales with the scales in sight so he himself can keep an eye on the numbers. Tell him to put weight on the affected leg and increase the amount on the scales under the affected leg and decreas under the other.

In general - lots of coaching, other patients in the gym encouraging, applause with positive results, aso. I have tried this on a few patients with log standing paralysis on the affected leg and it has worked in the end. Requires a hard working PT with great ambition AND a well motivated patient!

Good luck!