Advice needed: Boomer ate plums

On Sunday, Boomer was sitting under the plum tree. We noticed that he was eating a few of the plums that had fallen on the ground, but we didn't think anything of it. Yesterday when I got home from work, there was a pile of yellow vomit on the floor. Still we didn't get too concerned, because Boomer sometimes eats something that he shouldn't, and vomits. But last night Boomer just wasn't himself. He was just laying around, he acted like it hurt for him to jump on the couch or bed. He is just very lethargic. I am going to call the vet as soon as they open, but has anyone else had experience with plums? I read that the pits can become an obstruction if swallowed, and if chewed they are poisonous. I read both the words cyanide and arsenic. I would guess that he ate 2 or 3 plums, and I am not sure about the pits, but I did hear him chewing on them.