He used to eat about a tablespoon of seed per day, plus some lettuce/other veggies and some other 'treats' (bread/cereal/cooked rice/etc). Now he is eating about 2 tablspoons seed a day; I don't think he is eating anymore of the other things, but I haven't been giving him anymore than usual so I can't be sure. Actually, I did switch seed brands, it would be probably a few months ago, he just finished the bag of the new seed brand. I got the new brand because it had fewer "extras" in it that he never ate anyway. He pretty much just eats the millet out of his seed mix, and ignores the other seeds (sometimes he likes the flaxseeds). Now that you mention it, I wonder if the new food has anything to do with his eating more... but I can't see how there could be that much of a difference in the seed mixes; they pretty much have the same main ingredients (both brands I've fed him, the old one and the new one, had some pellets mixed in, but he never eats them).
I also have my doubts about it being worms; I don't know a ton about them, but it does seem odd that his poop would be normal (and from what I can tell, it doesn't look like there are actually any worms in the poop). And he is acting perfectly normal (except eating more, of course) and seems healthy. But I can't think what else might cause him to eat so much more.
He hasn't been in direct contact with any birds. I have taken him outside in a cage last summer, but I was watching him the whole time and no birds got near him (and I haven't had him outside yet this spring). But I do have chickens and feed wild birds, so I guess I could have somehow contaminated him if I didn't wash my hands well (I'm not sure if that's actually possible, though).
Thanks for the info, Finnie. I worry about my precious little birdie, lol.