My Belle was pretty sick after her spay. She couldn't keep anything down. I called my vet and he first suggested boiled chicken and rice, but that didn't stay down. I picked up a few cans of I/D from him, that came up, too. So we ended up at his office the next morning and he gave her and injection to stop the vomiting and sent us home with pepcid and sucralose. After the injection she didn't throw up again. (she didn't have any diarrhea, just the vomiting)
Its not normal for them to get so sick after surgery. Please call your vet asap. Hope your little guy gets better soon.