Again, you want B2(Riboflavin) for leg support.
Thiamine (B1) is essential as well, but Thiamine Deficiency looks like neurologial problems (tremors, stargazing, etc.).
The B-Complex usually contains all the B Vitamins (there are 8 of them), so if you can get that, it will get the B1 and B2...
Any lice or mites on the bird?
What are you feeding?
I'd start giving 1/2 tablet B-Complex daily. The B-Complex contains B2(Riboflavin) which is what you want for leg support.
I'd feed a 20% Chick Starter or an All Flock feed.