"I disagree with the people who are suggesting you hide it/replace it without him knowing though. Death is one of the toughest topics to explain to a kid, but something like this could be a great opportunity to do so in accordance with your beliefs."
That's touchy... i believe ABC said he's what? Three?. My niece is three and we tried that with her, but she didn't get it.
As an update I did end up telling his as best I could, he may only be 3 but is too smart and would have realized if I had simply switched a new fish in without him noticing. He got noticeably upset, and I think only somewhat understands, but at the same time I am glad I told him rather than lie to him.
I took him out over the weekend to pick up a new fish for him, he at first wanted angels (I told him they would get too big, so he pointed to the smaller angels and wanted them instead). We ended up getting a new betta that he has named "sally"