Billy joined the UBCS lol
http://residentevil.wikia.com/Billy

No, he got abducted by aliens

Truth be told: Nobody really knows what happened to him post RE Zero. His only real purpose was to help Rebecca in another typical horror movie 'male teams up with female' situation, and now that it is long over between them, I'm not sure that Capcom cares enough about him to acknowledge him.

Yes, they had to put him in The Umbrella Chronicles, but it's essentially just a reworking of the plot of most of the main games, including Zero. So it's not really new material about his character.

I think Tofu absorbed him o-o

Soy is a super food. Every fridge should have tofu!

I actually agree with you Pedro, although Billy had the potential to be a main stay, its already to late, and Capcom probably doesn't care for it. BUT I actually feel the same about Rebecca, she was a supporting character in RE 1 and was used in RE Ø only to back up the story that was told in RE. and she hasn't been used since, despite her appearance in The Mercs. So, if Rebecca were to get another role in the series, I would think Billy should deserve the same.

Barry is the same too, I guess. Hence why he's only in novelty games like Mercenaries or Extreme Battle, etc, that do not have anything to do with the story.

This.

Maybe he and Rebecca will meet again on a mission, not unlike how Ada found Leon in Spain.