Friend, it is common for a dog to develop loose stool or vomit after eating chunks of fat from a steak. But, you are right, if he ate the plastic this could be an indication of the plastic being obstructed. Unfortunately, on history only it is not possible to determine if he has plastic in is gastrointestinal tract, I am sorry. The only way to rule out is by taking x-rays with barium (contrast material). This is what I would do:
1- Do not give things like Pepto Bismol or Imodium. Stopping the loose stool with medication can make things worse if he has an obstruction.
2- No food or water for 6-8 hours, need to give his gastrointestinal tract time to rest. Then, offer a small amount of water followed by a diet of rice (75%) and boiled boneless chicken or ground beef (25%) until the stool is back to normal then wean him back into his regular diet. Keep him indoors in a cool room and no exercise when fasting.
3- Supplement with a probiotic, these are friendly bacteria that help with regularity. You can buy this over the counter in any pet store.
4- If after doing the above you do not see improvement or if he becomes lethargic, continues to vomit, develops abdominal distention, pale gums, etc. I would have him checked by a veterinarian for a hands on examination and x-rays with barium.
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