Worms and external parasites generally won't cause staggering, which is what he appears to be doing....not unless the bird is so emaciated that he is so weak he stumbles...but I seriously doubt that as he was fine one day and stumbling the next. Possible, but not probable. If it were the external parasites that you treated, his comb would be very pale and he would look dull and thin as well.
The first thing that leaps to mind is Marek's because he appears to stumble. That is not a good sign. I will link a really good BYC article on Marek's for you to peruse.
It could also be a soft tissue leg injury. Watch to see if he doesn't want to touch one of the feet to the ground (also check for bumble foot as it is winter and he is a foot feathered Cochin).
I had a similar situation with my Barnevelder rooster....absolutely fine and healthy one day, then the next severely limping. But he clearly didn't want to touch a foot to the ground, not desiring to bear weight on one of his legs, rather than stumbling...although it could appear to some that he was stumbling at first glance...which is why I offer this possibility to you. Watch him. Is this stumbling from weakness? Or limping to avoid putting down a painful leg?
With my Barnie, I determined it looked more like limping on a tender leg. He too had no appearance of broken bone, and the leg moved freely. He also was reactive in that leg, curling toes to touch and vigorously attempting to pull his legs away from me (a good sign). I looked around the yard and discovered a rolled ball of weed eater twine had mysteriously been dislodged and strewn....ah ha...he had been in that area. I strongly suspicion he had got caught up in it and strained a leg in frantic attempt to get out of it. He limped hard on that leg for about a week. I locked him up in the coop for a couple of days to give him some rest. After about 2 weeks he limped lightly. After about a month, I see no sign of any limping, and he has been fine and healthy. While it could have been a mild incident of Marek's, with the twine and no further sign of paralysis (no wings or neck affected), I have chalked it up to twine injury.
CRD, IB, MG and various other ailments can cause stumbling as the bird is ill, but in my experience you have other obvious signs of illness...coughing, wheezing, nasal discharge, imho. I've had birds with IB run around, coughing, but not stumbling. Only the birds that were really sick sat huddled and rumpled. They didn't stumble around.
My best guess right now is he may have strained that leg. I would isolate and give some TLC to see if a few days rests improves things. I'll link the Marek's symptom checker below, with botulism poisoning info, in case it could be either of those. Botulism comes as a second guess, or another such toxin, since it came on so quickly. Vitamin deficiency is also a possibility. Marek's, unfortunately, will be my front runner guess if the stumbling worsens and does not appear connected to a tenderness in a leg. (You can see if he still has good reflex reaction in the feet and legs).
Placing him on some vitamin boosts and electrolytes can help if vitamin deficiency. The Marek's article will include some good info for other treatments for Marek's mimicking illnesses.
My thoughts. I hope your boy is improving soon. Keep us posted.
Edited by Lady of McCamley - 11/25/15 at 1:35am