He should get that checked out. If he's had seizures, why haven't they done a neurological workup? He could have something causing adolescent onset of epilepsy. Seizures can have a variety of presentations, and if he's seizing in his motor and sensory cortices (they're right next to each other), that might explain the numbness and weakness. Another option might be really nasty migraine auras (they can occasionally mimic strokes), but I think epilepsy is more likely (note that I'm not an expert on epilepsy or any sort of doctor). Then again, if it takes days for the numbness and weakness to set in, I've got nothing. He should definitely see a doctor though.
At the very least he should get an EEG, which would identify abnormal brainwaves caused by seizure activity. An MRI or CT of his head might be in order to check for things like tumors, but keep in mind that brain tumors can cause just about any symptom. If his doctors won't believe him, try to get a video of him while he's having an episode to show them.
If the mood swings are independent of the episodes, they might be an entirely separate issue (ultra rapid cycling bipolar can cause mood changes over the course of the day). If they're with the episodes, though, they probably are related.
look up aura migraine or epilepsy? :)