I'm sorry to hear about your fellow's episodes of meowing, lethargy (sitting in one spot) and drooling, and then appearing tired compared to his usual self.
Is he responsive to stimuli during these episodes, meaning if you touch him or talk to him does he respond appropriately or does he seem very spacey, and doesn't respond?
I suspect that these episodes may be atypical seizures.
Typical seizures are rhythmic, repeated muscle movements which the cat is unable to control and can lose consciousness during. Many cats will fall, have repeated motions/tremors, may vocalize and cannot rise, drool, and can lose urine and stool control. They can be a bit weak or uncoordinated afterwards, and may seem confused but should come back to normal in a relatively short period of time. With atypical seizures we may see a loss of awareness, drooling, vocalizing, and an inability to respond appropriately to stimuli, but we may not see the rhythmic muscle movement or loss of continence.
There can be several reasons for seizures.
The most common is idiopathic epilepsy. That means that we don't know why but a circuit of sensitive neurons in the brain gets stuck repeatedly firing. Epilepsy occurs most frequently for the first time in young animals, and your fellow is relatively young so it is possible that he is epileptic. These cats tend to be very normal outside of their seizure episodes.
We do believe that there is a genetic basis for epilepsy as certain breeds are more commonly afflicted and siblings will often have them as well.
Other cause for seizures are viral, bacterial, parasitic or fungal infections, metabolic diseases leading to waste products building up and affecting brain chemistry, low blood sugar, toxin exposure, or even granulomas or masses in the brain. Most of the other disease processes that cause seizures cause other symptoms, those cats are sick or abnormal other than during the seizure.
Is he gaining or losing weight, drinking more water than usual, or exhibiting any strange behavior otherwise?
Most cats with epilepsy do behave normally after they gather themselves after a seizure, though they will often be hungry and thirsty and may feel anxious because they do not remember what happened during the episode.
Some cats with lower than normal seizure thresholds will seizure in response to being exposed to artificial colors, preservatives or gluten. So you might wish to feed him a diet without artificial dyes or flavors and one that is wheat free. Blue Buffalo purports to produce these sorts of foods.
Decreasing stress is also a way to avoid seizures so if you know an event will be stressful for him avoid it if possible. You can also use calming sprays such as Feliway or pheromone impregnated collars to keep him calm. Was there any stressful event that you know of that led to this episode?
Exercise should be kept at normal levels. Exercise is a great way to naturally relieve stress and increase positive endorphin levels in the brain.
Ideally he should see his veterinarian for an examination and bloodwork to look for underlying organ disease, feline leukemia or feline immunodeficiency virus as well as toxoplasmosis. We do want to make sure there are no underlying problems that need to be addressed.
If he has more than one seizure today, or if he is not eating and drinking or is behaving abnormally otherwise then an emergency visit today is best.
We tend to be more aggressive with diagnostic testing in cats to look for causes for seizures, and with treatment. If a cat has more than one seizure in a month or more than one in a day, even if it has been months since the last one I recommend medical therapy to prevent seizures.
The reason for that is the likelihood of status epilepticus (one seizure after another) and possible brain damage is higher with those scenarios and we wish to avoid that.
In a cat that has only one seizure episode, is behaving normally and has normal test results it is reasonable to take a wait and see approach. If he never has another there is no need to do anything. But if he has multiple seizures then it is time to consider medication.
Phenobarbital is very effective in most cats at preventing seizures and if used at appropriate levels it rarely causes any organ damage. If you aren't comfortable with that drug there are others such as Keppra (levetiracetam) or Zonegran (zonisamide) that your veterinarian can prescribe. These newer drugs are reported to be less stressful on the organs but even they are not totally without problems. About 1/2 of the cats on zonisamide do have side effects, including diarrhea, lack of appetite, vomiting and incoordination. I find most cats handle phenobarbital better long term after the first few weeks of adjusting to the drug.
As with any medical condition we must weigh the positives and negatives of using a particular medication. I would try and decrease his stress levels as well as using a diet without glutens, dyes or lots of chemical preservatives.
If your fellow has more than one or two seizures I think preventing brain damage by using appropriate medications at appropriate doses is more beneficial.
If medication is prescribed make sure that his blood levels are checked periodically so that we aren't under or over dosing him.
In many cases we never completely stop the seizures, but they should be much fewer and much less severe.
Please let me know if you have any further questions.