I know this is way too late for your assignment, but actually you asked what stage in the gait cycle is he likely to experience knee pain. I believe the answer would be Terminal Stance, or Heel Off, phase. Terminal stance is the point at which the hip comes forward from extension and the knee bends to allow the heel to come up off the floor. If he lacks knee flexion, this is, in my opinion, the first gait phase that he would begin to have considerable pain.