It sounds as if it could be pigeon pox. Can you tell us where you are and what sort of pigeon it is? Can you provide photos?
There is no cure for pigeon pox, but it is usually self limiting. You could give it echinacea to strengthen its immune system and vitamin A supplements. It could probably do with an antibiotic like Baytril to combat other infections and if it gets bad you could use a painkiller (Metacam is best).
Here is some information on Homeopathic remedies that can be used:
• Alternate Arsenicum 3X and Rhus tox 3X (3 pellets every 2 hrs.).
• Belladonna for hot fever and Silicea if pustules break. (2)
• If lesions on head and comb are wart-like give--Antim tart.
• If lesions in mouth are diptheric type give--Kali mure, Nat sulph.
• If lesions with fever give--Antim tart, Aconite.
• If lesions without fever give--Bryonia, Calc phos.