Poorly chick! Any advice would be lovely? (not for those eating...)

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My two week old chick has a very red and swollen anus. He doesn't seem to be growing as much as his brother either.

I noticed that this chick had a bit of diarrhoea the other day but now he seems to be a bit distressed and is cheaping quite loudly and not eating as much as the others.

I wonder if I need to take him to the vet or if there is something simple I can do to help him? He is not in a draught and is under a warm lamp and on clean sawdust.

TIA

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