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.
He's a big fellow and it may pass but if he just ate it, within the past 2-3 hours, it's worth trying to get him to vomit it back up.
You can give him 1 tablespoon of 3% hydrogen peroxide per 10 pounds of body weight mixed with bread chunks or dog food (no more than 3 tablespoons even if he weighs more than 35 pounds).
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.
If it has been several hours since he ate the cap or if he doesn't vomit it back up then feed a homemade diet of 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats and juices drained off the meat, and 2/3 boiled, plain, white rice. Add 2 tablespoons of canned pumpkin, (not pie filling, just pumpkin), to each meal for fiber to push the cap through his intestinal tract.
If you don't have peroxide you can get him to an emergency clinic to induce vomiting.
If it has been several hours since he ate the cap it is likely too late to induce vomiting, the cap will likely be out of his stomach.
All you can do at that point is feed the bland, high fiber diet and 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.