You said Tom has typical hemiplegic gait. Means he must have typical pattern of spasticity in lower limb. Which is pelvis retracted, hip in adduction, internal rotation and extension, knee in extension and foot is in planter flexion.
Well, to clear foot in swing phase, hip flexor, knee flexor and dorsi flexor should work. Missing of one of these componant may alter the gait. While in this patient the pattern is totaly opppsite to it.So he would not be able to clear his foot is obvious.
And in Hemiplegia, knee pain start when spasticity is severe and knee is not so mobile. Or patient has knee pain since his premobid stage.You said patient has stroke before 18 months. so may be he is in stage of 2 or 3 of spasticty. So may be you should work on that.