First, ask yourself whether your son is actually unhappy or whether you're projecting discomfort onto him. It's easy to think "the other kids have X, so he must feel bad," but children are much more pragmatic than we give them credit for. If he's not complaining about it and he's just fine with his old pencil case, he probably doesn't see it as a problem. Forcing him to change just to fit in could send him the opposite message: that what he chooses isn't good enough, and that he has to follow the crowd to make you feel at ease.
The real risk is destroying something he has now - his independence of choice. At 8, kids start developing their own taste and preferences, and sticking with them in front of their peers is actually a good sign of self-confidence, not weakness. Teachers don't say anything because it's genuinely not unusual: one out of twenty kids will have a different pencil case anyway, a broken one, a vintage one, whatever.
Instead, talk to your son without trying to convince him. Not "don't you feel weird?" but more like "how are you getting on with your pencil case, do you still like it?" If he says yes, that's it. If one day he decides on his own that he wants something different, great. What matters is that it's his choice, not a correction.