Lennox-Gastaut syndrome: This is the most likely diagnosis due to the combination of sudden loss of muscle tone (atonic seizures), developmental delay, and the characteristic EEG pattern of generalized slow spike-wave discharges at 1.5–2.5 Hz. Lennox-Gastaut syndrome is a severe form of epilepsy that typically presents in childhood with multiple seizure types, including atonic seizures, and is associated with intellectual disability.

Childhood absence epilepsy: Although the primary seizure type in childhood absence epilepsy is the absence seizure (a brief, sudden loss of consciousness without a post-ictal phase), some patients may exhibit myoclonic or atonic seizures. The EEG pattern can show generalized spike-wave discharges, but the frequency is typically higher (3 Hz) than in Lennox-Gastaut syndrome.
Juvenile myoclonic epilepsy: This epilepsy syndrome is characterized by myoclonic seizures (sudden muscle jerks), generalized tonic-clonic seizures, and sometimes absence seizures. While it typically starts in adolescence, some cases may begin earlier. The EEG shows generalized spike-wave or polyspike-wave discharges, but the clinical presentation and age of onset might not perfectly match this patient's scenario.

Temporal lobe epilepsy: Although less likely given the generalized EEG pattern and the specific seizure type described, temporal lobe epilepsy can sometimes present with atypical features, especially in children. It's crucial to consider this diagnosis due to its potential for significant morbidity if not properly managed.
Benign rolandic epilepsy: This condition typically presents with focal seizures, often involving the face and/or arm, and has a characteristic EEG pattern showing centrotemporal spikes. While it's generally a benign condition, missing the diagnosis could lead to unnecessary concern or mismanagement.

Myoclonic-astatic epilepsy (Doose syndrome): This rare epilepsy syndrome presents with myoclonic-atonic seizures and is often associated with developmental delay. The EEG can show generalized spike-wave or polyspike-wave discharges. Although it shares some features with the patient's presentation, it is less common than Lennox-Gastaut syndrome.
Other genetic or metabolic disorders: There are numerous rare genetic and metabolic disorders that can present with epilepsy, developmental delay, and specific EEG patterns. While these are less likely, considering them is essential for providing comprehensive care, especially if the initial diagnoses do not fully explain the patient's condition or if there are additional symptoms suggestive of a systemic or genetic disorder.