After seeing the details regarding the patient’s gait, it may be due to excessive coactivation of both flexors and extensors of knee joint during swing phase. He lacks reciprocal relationship between both muscle groups. If the case is like that try my ideas and I hope it will work.
Train his hip and knee flexion gait pattern in all 3 stages of swing phase (acceleration, midswing and deceleration).
To train initiation of gait (preswing and acceleration)- position the patient in step standing with affected leg behind with a bolster in between the legs, therapist’s position- kneel behind the patient with one hand supporting the forward leg pelvis and other hand over the shin of tibia of affected leg. Ask him to clear the ground and give him tactile cues through shin of tibia by sliding ur hand downwards and little backward to bend the knee. This will facilitate knee bending during initial stage of swing phase and also push off .
Midswing- patient stands on single leg with affected leg over a small stool and ask him to touch an object kept at reachable distance in front. This will bring knee flexion and extension in mid ranges, which will reduce coactivation of both muscle groups. You could train ball kicking in this stage.
Deceleration- same position as I mentioned for midswing but the affected leg is over bolster or roller skates. Ask him to roll it forward and bring it back. This will facilitate ankle dorsiflexion too.(check Hams length)
Note:It is not mandatory to train all in one session. Repetition is necessary to learn. For additional effect give pressure to the muscle bulk to activate it in correct timings. Ensure the patient balance and give additional support during training if he needs (parallel bar, stick support or others support etc). Don’t give resistance this may stimulate unwanted activation of other muscle group.