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This actually sounds like it could be a 'cyst' which is a collection of fluid under the skin- antibiotics will have no effect on it.
To confirm if this is the case take him to your vet for an 'FNA'-fine needle aspirate- and if clear fluid is withdrawn then it is a cyst- which he should be able to live with without it causing him an issue.
To find an avian vet in the US go to www.aav.org and use the 'find a vet in my area' link on the left of the screen.
try www.birdsnways.com/articles/abvpets.htm which lists some certified vets in Canada at the bottom of the list.
If it doesn't bother him then it may be best to leave the lump well alone- but an FNA will at least give peace of mind that its nothing to be worried about.
All the best Andrea