A lot depends on whether you think your son would intentionally try to fool the monitoring system.
As @AllJetNoPilot and @Jdberry mentioned, there are several ways that you can monitor activity that would be part of this process, and use the absence of that activity to trigger reminders. The contact sensor on the medicine cabinet or a drawer where the toothpaste is kept is a really good example of this, often used for people with Brain injuries or cognitive challenges, but also useful just with regular kids.
The issue with all of these is that if a child wants to fool you, they can usually figure out how to do it.
Depending on the age or temperament of your kid, the Foreo ISSA mini toothbrush may help. This is an electric toothbrush designed for kids which has a happy face indicator that turns on after you have brushed for two minutes. Then if you haven’t brushed again in 12 hours, it changes to a frowning face.
Reviews are all over the place, but a lot of the negative ones are from adults who tried it and found that it wasn’t as powerful as other sonic toothbrushes. But that’s because it’s intended for kids. Also this isn’t a spin brush – – you have to move your hand like a regular brush. But it does have a sonic component.
My dentist recommends these, and I trust him a lot. If you want to try it, buy one of the colors which is sold directly by Amazon and it should cost a little less as well as have a better return policy if it doesn’t work for you.
I would probably combine a contact sensor on the drawer with the toothpaste and the foreo. Start with gentle reminders and see how it goes.
When I was a kid, my mom would also do occasional spot checks (literally!) With the Butler GUM chewable Red Cote tablets that leave red spots any place you haven’t brushed. These also stain lips, though, so you only want to use the last thing at night so they’ll be clear in the morning. I would only buy these at a local store, though, not online, to make sure you’re getting the real thing and not something that’s expired. These take more time because you have to watch the kid use them, but as an occasional check I think they can add a lot to a routine. I know they did at our house (four kids).