Lump on my bird. What should I do now?

Hi! I have a 11 year old male cockatiel. He developed a large lump on the side of his neck. But otherwise was fine. I took him to the avian vet (about a two months ago) who drained a bunch of nasty brown fluid from it. But the vet didn't know what is wrong. I gave him some antibiotics twice a day for a couple weeks. He seemed a little more chipper but that may be because the lump was reduced and not in his way. The lump has gotten just about big as it was before it was drained. Today I notice that it seems to be itching my bird. At least he keeps trying to scratch it.

I don't know whether to try and have vet drain it again or what to do. Any suggestions?