The term epilepsy really means the tendency to have recurrent seizures, and it does not imply what the underlying cause of the seizures is. Likewise LGS is a descriptive term of epilepsy, multiple seizure types, and an abnormal EEG, and does not imply what the underlying cause is.
I cannot be more specific about what your sons diagnosis is over the internet, but if not done already, further testing might reveal what caused the epilepsy in the first place eg MRI of the brain, and careful analysis of the EEG, and selected metabolic testing if the MRI is normal. Sometimes even with multiple siezure types a focal abnormality such as a localized brain malformation may be present (that is, an isolated part of the brain is normal, and the other parts are normal) - this would have important implications for future treatments.
The Clevland Clinic Pediatric Epilepsy Center has a lot of experience in cases like this and could provide an expedited workup and second opinion if you so wished.
Good luck
Also all testing with EEG and MRI are clear