If you mean SanDisk, it looks like there are a couple of models that they offer, so I can’t offer you case suggestions without knowing the name of the model. If you can find which one it is, I will be able to help you better.
However, while looking at the players, I discovered this case, which is reasonably priced and is designed to “guards against minor drops, scratches, bumps and dirt.” If that fits the model that your son has, it might be a good option.