I am no vet, but even if you can't visually tell it's hurting him, you really don't know whats going on inside just yet. It seems harmless, since it's just seeds, but you never know.. I would just try to have him refrain from eating it, since someone above mentioned pancreatitis can occur.
just to be on the safe side, i'd cut him off...
maybe he wants to be a bird? hehe