Hello, I'm Dr. Deb. I'll do my best to help you today.
I'm sorry for this concern for Leonardo.
1. The first thing I'd want to check would be to see if he is running a fever. Normal is between 100 and 102.5. If it's elevated, then we'd have to wonder why since there are a number of possibilities:
Early upper respiratory infection
(usually secondary to herpes)especially since he has a small amount of discharge from his eyes.
Other viral infections such as FIP or Feline Leukemia or Feline AIDS (although this is fairly uncommon in a cat this age).
There's a condition called Fever of Unknown Origin (which is exactly what it sounds like) but this condition typically doesn't affect kittens this young.
I2. f his temperature is normal, then I'd want to rule out systemic disease which might be secondary to a birth defect. Kidney
issues, for example, don't just develop in older cats; kittens can develop them as well.
3. Similarly, underlying heart conditions can be seen in young kittens, too, although it may take advanced testing such as an ultrasound to determine if this is the case.
4. If he's anemic, he might be more lethargic. Anemia in a kitten this age could be secondary to a virus such as Leukemia but a flea transmitted disease such as a Mycoplasma (aka Feline Infectious Anemia) can also cause it. This latter condition can be treated with tetracycyline-based drugs and most cats will recover just fine.
So, as you can see, there are a number of possible explanations for his behavior and it may be difficult to narrow down the list unless you can take his temperature.
I don't typically advise over the counter medications for kittens (or more mature cats either)since they can be sensitive to so many of them which might cause more harm...which is the last thing I'd want.
But encourage him to eat and drink as much as you can by offering him smelly canned food
or even human baby food (just avoid those with garlic or onion in them).
If he continues to remain lethargic, then a vet visit may be prudent. I hope this helps and that there's nothing too serious going on with him.