If he was neutered last week this is not going to be an issue. Rice crispy treats are non-toxic to dogs. As with any dietary indiscretion, it is possible he could develop vomiting, diarrhea, etc. otherwise very likely he is going to be fine. I would keep an eye on him for the next 24 hours, if he develops vomiting or diarrhea this is what you can do:
1- Do not give things like Pepto Bismol or Imodium. Start him on Pepcid AC (famotidine) 10mg tablet every 12 hours for 2-3 days. You can buy Pepcid AC over the counter in any pharmacy.
2- No food or water for 8-12 hours. Need to give his gastrointestinal tract time to rest.
3- After 8-12 hours offer a small amount of water. Wait half hour, if no vomiting, offer a small amount of rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day. Keep on this diet for several days then wean into regular diet.
4- If he continues to vomit after doing the above he will need to be seen by your family veterinarian for a hands on examination.
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