I've done cold turkey changes with food and if it's right, they may go a shade soft but straightens up in a day or two. If the food is right, their poop should be fine.
Stop the liver treats and go easy on the milk bones
If you are seeing worms, they are tape or round and panacur will not take care of tape. If you are unsure of what you are seeing, take another sample to the vet with the white things in it!
Try boiled meat/chicken & potatoes for a day or so and see if he straightens up.