(For those who know Hebrew, I occasionally included the original Hebrew phrase for added clarity.)
Enjoy!

(The other Sons are: the Wise Son, the Wicked Son, and the Simple Son.)
Because he is not even aware that he needs to ask (or because he does not know how), he doesn’t say a word; he is silent.
Fascinatingly, the Kli Yakar does not take this Son’s silence at face value. He asks the Abarbanel’s question:
"How can it say that the Son is silent because he doesn’t even know to ask?
Perhaps he is silent because he is completely wicked (rasha gamur) and doesn’t even want to speak about this mitzvah because it so despised (bazuyah) in his eyes?"
So how can we know the real reason why someone is silent?
Needless to say, the Silent Son tragically describes the vast majority of Jews today—across all backgrounds and categories.
Some truly don’t know to ask. For one reason or another, they are literally unable to ask.
And some just aren’t interested because they despise the Torah values that don’t fit in with their liberal secular mentality or their unknowingly Esav mindset (i.e. addiction to materialism and wanting to be in control and attributing success solely to one’s own efforts rather than to Hashem).
So how should we handle this type when we can’t truly know what’s going on underneath the surface?
Start Off Soft 'n' Gentle
In order to find your way out of this confusion, go and say to him, "It is because of this that Hashem did for me in my exodus from Egypt."
Then, no matter what, you have fulfilled your obligation. Because if he is wicked at heart (rasha b’lev), you have set his teeth on edge….and if he is not wicked, and simply does not know how to ask, then say to him this same verse…. because if he had been there, he certainly would have occupied himself with this holy service (avodah) [and merited Redemption].

He notes that the word used — l’hagid (to tell) —sometimes indicates gentle speech, but other times, it indicates words “as hard as sinews.”
The Kli Yakar states that if such a person truly doesn’t know to ask, then you should “draw him to the service of Hashem” by using “gentle words that attract the heart of a person.”
If the First Method Doesn't Work, Then Go for the Double-Whammy
And the Torah’s opinion is to first test the Wicked Son to see whether it’s possible draw him into the service of Hashem, using gentle words (devarim rakim)…..And if he “stiffens his neck” and will not listen to “a voice of whispers—kol melachashim” (indicating a gentle tone in addition to soft, gentle words), then….feed him words as harsh as sinews, which will set his teeth on edge and tell him:
"For the sake of this [the worship of Hashem], Hashem did for me"— for me and not for him.
The Hagadah says, "Do not be intimidated by the Wicked Son."
"The tipping point is (ikar hadiyuk talui) whether he is a servant of Hashem OR whether he is not servant of His."
This is an incredible insight and profound advice.
In summary, the Kli Yakar is saying that you should give the Silent Son the benefit of the doubt and use “devarim rakim--soft (or gentle) words” and a soft tone of voice. And if he truly doesn’t know, then this approach will draw him closer. But if he is silent out of apathy and loathing, then these soft and gentle words will seem "hard as sinews, which set a person’s teeth on edge."
And that is how you can know exactly who you are dealing with.
Needless to say, this is EXACTLY what is going on in the world today.
Bringing It All Home
There weren’t really secular Jews in the Kli Yakar’s time — well, very few, anyway.
The Silent Sons of the Kli Yakar’s time would have seemed pretty frum by our standards.
So if I understand this commentary correctly, any Jew — whether secular or frum — could either be silent out of innocent ignorance and the inability to even ask OR silent out of apathy and loathing.
How we respond to soft, gentle words of beautiful Truth reveals everything.
Needless to say, anyone can have a kneejerk hostile reaction upon hearing the Truth for the first time.
That is actually pretty normal.
There are also people who argue as a way at getting at the truth.
That is also perfectly acceptable.
But there are definitely people who reject and rebuff such words; they even physically turn away and refuse to listen, and do so consistently, whether they are outwardly frum or not.
Now, taking this one more step further, I couldn’t help thinking about how would I know which Son I am?
After all, being honest with yourself is one of the toughest challenges.
So I want to say that even if you suspect that you might be a seethingly Silent Son (which is basically a Secret Wicked Son), you can always switch over from a seething Silence to a receptive Silence.
No physical action needs to be taken; the switch is all in your mind.
However, the above is mostly my own translation and therefore, any errors are also mine.
Rabbi Shlomo Ephraim of Luntschitz (1550-1619) lived in Bohemia (which is today Poland and Czechoslovakia). He served as rabbi and dayan and wrote several books, the most well-known being his commentary on the Chumash known as the Kli Yakar.