what's the tension like in hamstrings and why is there no push off? Is it due to lack of gastroc activity? I'm assuming that he is hip hitching to achieve swing through...correct me if I'm wrong.
Does he achieve heel strike?
It would be helpful to know what's happening at the hip & pelvis too. Especially active hip flexion in standing.
It's difficult to know what might help/be causing it without more information....

What is he like kicking a ball - does he fully extend the knee doing this or does he hold it in the same position?

Sorry - more questions that answers at this stage!