
I think if he were my dog, if it persists, I would take him back and possibly get the vet to do an xray/endoscope incase he has something wedged further down in his tum.
If they have eaten things they shouldn't have, it doesn't always affect them straight away. Earlier this year one of my bitches ate a Mcdonalds type paper cup with the waxy surface and about 3 days later, she was sick off and on for a day and I took her to the Vet and luckily she had actually chewed it up in places so although it wouldn't degrade in her tum because of the wax, it was passing through, but our Vet thought that the lining of her stomach was inflammed. At that time she didn't have the poops but when it started passing through her intestine, she did have a couple of upset tummy days.
Is he upto date with his wormings too?