recovery from ACL & Meniscus tear operation

Six weeks ago my 25 year old son had key hole surgery to mend a medial meniscus tear and a replacement ACL using the ligament from below the knee.
He has had 4 physio sessions and is able to cycle long distances and drive but he cannot walk!
Physios reassure him that this is because of an imbalance of musculature but we are both finding it extremely frustrating.
When he walks his knee locks out and he just can't get it to bend without severe pain.
We are told that this is because the patella is not tracking properly. If he holds it, or tapes it up less than 1cm to the right or left the walking movement is fine!
He is doing plenty of excercises to strengthen the muscles around the knee,and working through the pain barrier , but I am just wondering if any one else has come across this specific problem and how long it is likely to go on for.
We would certainly appreciate any help here.
My son is a pro snowboarder so very fit & was working as a cycle courier prior to his op.
He still has swelling round the knee, but the physios say this is not excessive.
We don't expect miracles and are well aware that this op has a lengthy recovery period, but I feel if he could just get over this hurdle he would be able to progress better.
Is it normal not to be able to walk after 6 weeks?