He is a few months old (~4). The problem started suddenly. The flock spent the most of the day inside the coop on Saturday due to rain, and the next day he started having this problem. Another hen had the same problem for 2 days, but she is fine now. In terms of food i give them bran, and pre-made food that also contains vitamins, and is high on protein. Could it be from this? The food is for more developed chickens, and for some reasons he is not that developed like the others of his age.
Debris in a compost can kill a chick/chicken
If the compost pile is not managed properly, the food can rot and certain bacteria can convert the foodstuffs in a manner that it will contain the botullin toxin. If the chickens get in there and eat the foodstuffs with the toxin, they can suffer and/or die.
Do a search here at BYC on Botulism and you can learn all you need to know. But yes, it's a real danger.