that may be a dried cecotrope. bunnies make 2 kinds of poop, the normal ones are the round things that you see in their litterbox, and the 2nd type is cecotropes. rabbits have short digestive systems, so they need to re-ingest some of the nutrients to fully absorb them. sometimes if your bunny is startled, they can abandon them and not eat them.
I thought that at first too, but there are other ceceotropes around and they don't look that big. I'm thinking I should take him to the vet, just in case
That doesn't look like rabbit poop, it looks like a piece of dog food If that was the only one, I wouldn't be too concerned. It was probably a dried cecotrope like @Sam_ said.