Hey there,

To me it sounds like he's partially subluxed his patella if he is tender on the border of the patella and probably bruised or done damage to the cartilage in the patella. The pain has inhibited the muscle activity around the knee hence a feeling of instability with weightbearing.

I would do a scan anyways to hopefully clear the knee of ACL rupture. Sometimes I have felt a normal ACL on testing but scans have shown it has completely torn! Sometimes the hamstring is still active at rest and can affect the testing.

in the meantime, treat him conservatively in the meantime with your general knee strengthening stuff and work alot on his VMO/quads/core stability (as pain permits) and progress as required. Should probably take 6-8 weeks of rehab for his knee.

Hope this helps. all the best with the patient.