Hello and thank you for using this site to help you.
I would hold off on the imodium for now and I would fast him for 24 hours (since last meal) then start your bland diet. You can try giving some plain canned pumpkin now as it is a bit more natural to give for diarrhea. Try two tablespoons.
The reason we fast for 24 hours is to allow the intestines the time is needs to become less inflammed.
The bland diet of chicken (no skin, no fat, no grease) and cooked rice is usually what we recommend. To start give only a couple of tablespoons then wait four hours, if he holds it down then give him a few more tablespoons.
I after the bland diet is given and you want to try some imodium I will direct you to a link on dosing . http://www.veterinaryplace.com/dog-medicine/imodium-for-dogs/
If the dog eats any non food items such as raw hides, bones, toys, greenies or other teeth cleaning type items then you may be looking at a blockage of the intestines and that would require an x ray and maybe surgery.
If the dog eats fattening human food then you could be looking at pancreatitis which can become serious.
And if the dog got into any garbage then you are looking at possible garbage toxicosis where there is moldy food and that too is serious.
If you know for a fact any of the three have occured please do not delay seeing your vet.