How would anyone know? If there was, it could never be made public because of his age. Many people believe there was- and that is why the Grand Jury did not return an indictment or even issue a statement as to why they did not (which they are allowed to do).
Your question is a riddle. It has no answer. If there WAS evidence, no one would know. Consequently- if there is NO evidence, we can't be sure there really is NO evidence, or is it simply a matter of evidence that cannot be considered because of his age.
That being said- unlike his parents, whose fibers are right ON the body, there were no fibers linking BR to the crime scene of which we are aware. The secrecy shield in Colorado may be hiding that information as well. There may still be some redacted information in the evidence lists (and I believe that is the case). The only thing we do know is that his fingerprints are on the pineapple bowl and on an empty glass with a tea bag in it. We don't know whether he drank tea from that glass or of the glass was simply used to place a used tea bag in it. I do not think the little paper tag on the tea bag was tested for prints or DNA. And there is no real way to prove the tea was consumed (if at all) at the same time JB ate the pineapple. Just because they are on the table together doesn't mean you can prove BR drank it. He may have been at the table with JB when she ate the pineapple, and to me, THAT is the kind of question the police need to ask and THAT is the kind of information he doesn't seem to want to give. The reason? If he admits being there when she ate it, that pretty much proved his parents were lying about the pineapple. Could they have been there in the breakfast room without their parents' knowledge- sure, but I don't think so. The timeline the autopsy PROVES indicates she had the pineapple between 10 and 11 PM. Patsy admitted being up late getting things ready for the trip the next day. JR admitted working on a toy with BR AFTER they got home - around 9:30- 10 PM. You see- THIS is why the pineapple was lied about. It isn't the pineapple snack itself. It is the TIME she ate it.
BR could provide at least an approximate time for the pineapple snack, if he was there. It is entirely possible that Patsy gave JB the pineapple during the time JR was helping BR with that toy. Or BR may have also eaten pineapple. That is something only he can say. And he won't say.
I have to say that I am not a fan of laws that forbid this information because of a person's age. Obviously, children below a certain age cannot and should not be prosecuted. But I believe the public has a right to know about these kinds of crimes, regardless of the age of the person involved.