Not silly at all! Keep asking! Yes, "he keeps doing" (or maybe in this context "keeps on doing") means something a little different from "He is still doing ...". Firstly, it is not limited to here and now, and secondly, it has an idea of persistence, even stubbornness, when time is short or there are other things to do. "He keeps [on] going out in the evenings, even although he knows he has to study more." The best way to think about the different meanings of "yet" is to figure out what part of speech it has, because those meanings are quite different. In your first example, it is an adverb modifying "has eaten" and it means "up until now". In your second, it is a conjunction and it means something like "however", but perhaps with bit more of a sense of contrast and apparent contradiction. Accordingly, it doesn't quite fit in your example (and also note you should probably put a comma before it). Here's another example: "She speaks fluent English, yet she didn't learn it until she was 30".

Oh ! Amazing ! Thanks Georgia ! I got the "still vs keep" part and about "yet" it helps like you have no idea ! Thank you for you patience and have a good day !

"He's still doing his homework" - a continuous action. He hasn't finished his homework yet. "He keeps doing his homework" - implies a repeated action. As in, he does his homework, then does it again, and then does it again. Another example: "You're still seeing him" - continuous action. "You keep seeing him on Saturdays." - repeated action. As for yet, that has a couple of meanings. "He hasn't eaten his lunch yet" - adverb of time. Used when something hasn't been done, but likely will do. "He hasn't eaten his lunch" on its own just means he hasn't eaten his lunch. But when you add yet, it sounds like he will eat his lunch at some point. "So I went down and found him yet he was unconscious" - not to do with time. Yet in this sentence acts a bit like 'but'. "I went down and found him but he was unconscious" has the same meaning. So if you get stuck, try switching 'yet' for 'but' and seeing if it still makes sense. Hope that helps!