I let mine sit in the fridge for at least 2 weeks. That mellows out that kick, but still a good strong flavor. I've had mine for months and it's still awesome. Prepared mustard stays good for a very long time. I've done a sugar free version with Truvia and a honey version. Just add to taste.
Guess the entire "fermented" in the beginning was a misconception since this is not fermented. Now there's an idea... With that amount of vinegar in it the stuff going bad is not likely. I bet this will last much longer than 2 weeks in the fridge. It's probably at pickle acidity levels. Plus it's easy to see when it goes bad: it either starts bubbling or is grows mold.
I understand where your son is coming from. I so dislike mustard. I can handle a little bit in potato or macaroni salad or a little bit in a peach or apricot glaze on meat but just a little bit.
This was informative I’ll now try making my own at home & without any ingredients I can’t pronounce I might add some horseradish. Wonder if it would freeze well?
If I was going to can it, I'd follow a recipe intended for canning. This will do fine in the fridge for a long while. And it's so easy to make. I think it would be best to make it fresh when you start getting low.