I'd say a respiratory infection is most likely. In our experience cockerels are most prone. Can be caused by dust in the coop or by condensation or wet bedding causing fungal growth and the release of spores. We had a cockerel that needed a double dose of Baytril for 10 days to clear it. Poor ventilation in the coop can be a reason (was in his case). Also coops with parts made of plywood or plastic because they don't 'breathe'.
You need to act quickly Dagirlie, because permanent lung damage can result from respiratory infections and they they are always prone to them. Clean out the coop thoroughly and wash any suspect mould areas with poultry shield (don't use Jeyes Fluid -the fumes are too toxic). Put in dust extracted and dry bedding. Mostly these things clear up on their own within 48 hours if that's the problem. If he gets worse though you will need antibiotics prescribed by the vet.