Zachyy, clearly explained why a 14 Pro Max, vs a 17 Pro Max operate differently as it relates to Siri. But your son can easily avoid the Un Huh response on his phone. If I said your name and then I paused after saying your name, you'd like say Uh Huh or something similar to let me know you heard me call your name. But if I said your name, without pausing and simply asked my question, you'd answer my question.
Siri on older devices is no different. All your son needs to do is train himself to NOT pause after waking Siri.
If he says "Siri......[pause]............Whats the temperature outside - Siri will say Uh Huh.
But If he says, Siri what's the temperature outside without a pause, Siri will answer without Uh Huh.