Simply put he's having trouble straightening his knee all the way or with full strength.
This is a common problem with ACL repairs using the patellar-tendon graft.
A small piece of tendon from below the kneecap along with a piece of bone on either side
of it can be harvested and used to replace the torn ACL. The quadriceps muscle along the
front of the thigh straightens the knee. It's attached to the patellar tendon. Removing a
piece of the tendon can disrupt the way the muscle pulls.
This problem is usually taken care of with a good rehab program. The fact that your son
is aware of the deficit is a good sign that his rehab team is working on it.