The first question is does he had full extension passively, without pain?

Secondly do you feel that this is weakness or pain causing the lack of extension. If it is pain then maybe address pain control measures, e.g. iceing first and during the work or meds.

Can he get on an exercise bike and ride with the seat quite high, or is there access to hyrdo at this time? The time frame is most unusual so I would suggest either there is a complication or it is psychosomatic in some way. Has the swelling now resolved or are there still signs of oedema or an effusion. The more info you can give on the assessment the better guided the smawer will be.

I have used gentle, open chain muscle stim with active assisted extension and ice successfully for knee ops to get early end of range extension, especially in ACL where the patient might be a bit timid. Sounds a bit barbaric but this often got the brain to understand that extension is in fact OK. I hope you see what I am saying here (i.e. not to blow of the patella tendon after a graft has been taken from it, just get still to the quads).

Anyway closed chain is always the best option so you can try mm. stimulation with a host of closed chain activities with the good leg assisting.

Looking fwd to hearing more on your assessment.