Zigbee, flashing lights, but also noise. There's three volume levels but no way to have the lights flash without some noise. The lowest is pretty tolerable, not quiet but not ear piercing.
The lights aren't exactly blinding, bright-ish, but not like you'll notice them if you're facing the other way and don't hear the alarm.
But if you can put it in his line of sight, done for just under US$22.
https://www.aliexpress.com/item/4000813903996.html
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