I'm sorry your chick is ill. How long as it been like this?
The abdomen looks swollen too.
My first thought was omphilitis (infected naval cavity) from hatch that left unchecked causes additional swelling, called "squishy chick syndrome" meaning it has a rampant bacterial infection. It needs antibiotics immediately if you wish to save it.
Other causes of swollen extremities are equally grave. My second thought, since the chick is 2 to 3 months old, and if it had no lingering symptoms from hatch, is Infectious Synovitis, which is caused by a specific bacteria often in the soil, causing swelling of the hocks and inability to walk.
You can read more about it here:
http://www.poultrydvm.com/condition/infectious-synovitis
My thoughts.
LofMc