Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that your pup ate something he should not have, a plastic bottle cap.
It may pass because he is a very large dog, but if he just ate it, within the past 2-3 hours, it's worth trying to get him to vomit it back up and not having to worry.
You can give him 3 tablespoons of 3% hydrogen peroxide mixed with bread chunks, a small amount of dry dog food, or ice cream.
Then take him outside and run him around to get the peroxide bubbling so he vomits.
If no vomiting in 10 minutes repeat the peroxide dose once.
If he vomits and seems to clear his stomach and be done I would check to see if the cap is present.
Once he finishes vomiting and empties his stomach to try and settle his stomach today you can give either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle his stomach, and reduce any irritation the peroxide causes to his stomach lining.
Then feed a homemade diet of 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats drained off the meat, and 2/3 boiled, plain, white rice. If he does not vomit the cap up, or you choose not to induce vomiting Add 2-4 tablespoons of canned pumpkin, (not pie filling, just pumpkin), to each meal for fiber to help push the cap through.
If you don't have peroxide you can get him to an emergency clinic to induce vomiting.
If he doesn't vomit it back up or you choose not to take him to an emergency clinic still use the bland diet with pumpkin to try and get the cap to pass.
If it has been several hours since he ate the cap it is likely too late to induce vomiting, it will likely be out of his stomach. All you can do at that point is watch him closely. If he starts vomiting, has a tense painful belly or refuses to eat or runs a fever (more than 103F rectally) then that can signify a blockage and he should be seen by a veterinarian immediately. Surgery may be needed at that point.
Please let me know if you have any further questions.