Wow, yes, it's hard to even approach this one. Yes, your son is eating the toothpaste, which contains saccharin, which is a carcinogen and a poison. Exactly how much poison is he comfortable with your son eating?!
There are so many things that are nearly impossible to avoid, noxious air, poison in the water, bacteria in food, artificial colors, fats, etc, etc that it seems like anything that is easy to avoid, like this, is well worth the effort.
You could always tell him that I am a dentist, and I won't let my children touch the stuff. I also don't like it for myself, since I'm not sure how much gets absorbed through the mucosa (that is, after all, how pharmaceutical "patches" work, as well as nitroglycerin) and I'm not even swallowing it.