Siri not working with hoarse low voice

My father has a very hoarse voice caused by a laryngectomy. Although his words are audible and spelled out well, Siri doesn’t allow him to configure “Hey Siri” on his new iPhone. 

The weird thing is that if he activate Siri by pressing the Home button, she’s easily able to understand and respond to my fathers voice. This is causing him problems because he used to make calls and send texts by activating Siri during his motorcycle trips.

I hope this issue will be taken into account as a matter of accessibility for those users who had their vocal cords removed or voice-related surgery. 

Here is a video that provides quite an accurate idea of the voice.