This is a long shot,but check for a slipped tendon. Run your thumb down back of leg from where leg joins body to where scales start,press down while doing this,if tendon has slipped in one or both legs(slipped tendon in one leg can cause birds to not walk)you can feel it,it will feel loose/springy almost like an elastic band. Press down you should feel it snap back into place,this may have to be done several times a day or wrapped to keep into place.
I have successfully treated a slipped tendon in an adult rooster,his symptoms were not walking,but hobbling/crawling very low to ground and using wings for balance,he also was not eating much until i realized what the issue was(i was originally treating him for a sprain,but after a couple of days i knew it was something else). It took him about 2 weeks before he was back to walking normal. I pressed tendon into place several times a day and did not limit his mobility as i did not want tendon to stiffen up,i also did stretching exercises by gently stretching leg backwards like birds do while normally stretching.