Damaged intestines?

A few weeks ago I busted Diesel eating berries off a tree in the back yard. I told him no and I have kept a close eye on him while outside since then. That night after he ate the berries he began having TERRIBLE diarrhea, even in the house (very unusual for him). This lasted a few days, I became very concerned and took a sample in to the vet. The vet checked and there are no worms in his stool or any other concerns. We also started him on heartgard just in case. That evening I started him on rice, beans, and hamburger. After a few days of this the diarrhea was gone! So I slowly introduced his 4Health puppy food (Lamb and Rice), and the diarrhea started again right away. So back to rice, beans and hamburger. I went out and bought Blue Buffalo Chicken and Rice puppy food. The food is a huge hit and the diarrhea went away, hooray! After a week and a half of Blue, yogurt, carrots and smart bite treats I made the mistake of feeding him a Beggin Strip. Now for two and a half days he has had diarrhea again. I threw away all treats but the ones that he had been getting with no diarrhea. I don't mind feeding him natural food and treats, but my question is did the berries damage his intestines? Or what is your opinion on this situation? Your response is appreciated!