Do you usually use calcium/vitamin D 3 powder? Does it look as though he has any injuries to the leg? Is he moving the leg at all? Do you notice any twitching or odd movements of the leg or foot? Are the rest of his limbs moving normally?

I would gently move him to a quarantine tank with paper towel substrate and mist. Give him a shallow water bowl he can't get hurt in and can easily get out of.

Definitely dust his crickets.

The treatment will depend on if the leg/foot in injured vs. vitamin deficiency. Since he is clumsy right now, this is the safest way to protect him. But try and stress him as little as possible.