Dog yelps when pooping

[Mojo.jo.jo]

I have a 7 year old Yorkshire terrier.
In 2019 he started to yelp when pooping.
He had xrays, ultrasounds and in the end was told he could no digest his food properly.
So he was feed Royal Canin Veterinary Diet Canine Hypoallergenic Small Dog Dry.
He was OK till 4 month ago it’s come back.
The vet says there is nothing wrong with him.
I have to find another food for me or put him down.

I have tried almost all the dog food in the shops.
Has anyone got any ideas on what else I can do?