Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned as you found your fellow with an open bottle of Dexilant (dexlansoprazole) and he may have ingested a pill. Was this the 30mg pr 60mg size?
This drug is a proton pump inhibitor, like omeprazole (Prilosec) that decreases gastric (stomach) acid production. These drugs are fairly well tolerated in dogs and in most cases even with a one time slight over dose we don't see any toxic effects, or they are mild such as loose stools and a decrease in appetite for a couple days.
The no-observed-adverse-effect-level (NOAEL) seen in a dog study was 5 mg/kg, or 2.27mgs per pound.
Your fellow is likely pretty big being a Collie.
If he ate the 30mg size and weighs more that 14 pounds (very likely) he will be fine.
If he ate the 60mg size and weighs more than 28 pounds (also very likely) then he will be fine too.
I don't think you need to do anything special for him, and it is pretty unlikely he will show any signs at all if he did eat one pill.
Please let me know if you have any further questions.