Knee, Hip, Calf, Thigh Pain, Limping, No Known Injury
Honestly, I recommend you bring him for an evaluation as soon as possible. If the dysfunction of his leg is such that now he cannot even bear weight or walk normally in addition to not being able to straighten it then a more immediate evaluation is warranted.
The reason why I suggest having him evaluated sooner is to make sure there is no some more immediately reversible cause of this problem which could get worse with 2 more days of waiting. We have to be concerned with the nervous system, the musculoskeletal system and the vascular system in this case.
It is difficult to speculate in the absence of being able to examine him right now but it could be something like a ruptured tendon, torn muscle or some kind of juvenille arthritis; although it would be rare to see an arthritic condition affect only one leg. I would minimize his weight bearing and walking until this is looked at by a physician. No need to wrap the knee for now.
A good physical exam and possibly an xray will help yield more clues as to what may be going on and doing this sooner rather than later makes much more medical sense.
I thank you for your query. I hope you have found this answer to be adequate as well as informative. Should you have additional concerns, I am available to address them.
Sincerely,
Dr. Galamaga