You can hear it. Also if you hold the handle of the pot, you can feel the vibrations caused by the boiling action.

I don’t know how well they work, but there are things you can buy that if you place in the water, they will rattle around when the water is boiling.

The bubbling sound.

Also, if you have a clean dry plate you can hold it above the pot (VERY CAREFULLY. and not too close) for 15 or 20 seconds, and some of the steam will collect and condense on the plate, and the plate will be have water on it. This definitely will take some practice. This is much less safe than listening for the bubbling, though.

In my experience, a boil is much more bubbly sounding than a simmer. It just comes with practice though. Lots of repetition. A little bit of getting it wrong before you get it right.

Apart from feeling the pot or handle for vibrations, another thing you can do is take a wooden spoon and rest it on top of the water surface, with the middle of the spoon handle resting on the lip of the pot. You lightly hold the end of the spoon, basically creating a lever, and you can feel the spoon bobbing up and down as the boiling water pushes it about. If you can supervise, have him set up the spoon at different levels of water temp, like what it feels like when starting out, how the spoon moves in a simmer, and how it feels when you get a roiling boil, that way he can memorize what each stage feels like.

I don’t have any suggestions, just want to say that a lot of cooking depends on one’s hearing when Blind

If you put your hand just above the pot you can sometimes feel the steam from the boiling water. Also the vibrations in the handle sometimes and the bubbles

I just grab the handle of the lid and push it down, so I can feel it. I guesstimate the moment by hearing.

As someone with one seeing eye: that method is easier than actually lifting the handle off.

Hmm I never needed anything but just the sound of the pan, but if there's a hearing issue too, I guess that's tricky. Tech to the rescue!

That's great!