For a "jello" substitute, if the "plain gelatin" doesn't contain sugar, you could juice your own juice and then use the juice with gelatin to make "jello." I saw a really cute idea--you juice oranges, and then you slice the peel into quarters, clean the quarters out, and set the jello in the peel. Small enough for little hands, and fun to eat.
I'm allergic to one of the dyes used in that licorice mix I used to love but can't eat and of which I can't remember the name (I think it was the red dye used for the "berries"--I miss that licorice and am so tempted to buy it whenever I see it). I didn't know this until I was almost 30. But, this was one of the reasons why I stopped buying pre-packaged stuff and have been cooking from scratch since. There are things you can make that don't take hours. Good luck on this adventure. I know it seems overwhelming, but once you get the hang of it, it won't be so hard.
You can grow stevia in your herb garden. I dry mine. A little goes a long way. I add ground powder of stevia to things I want to sweeten. Fortunately, I'm not big on sweets. Is your son allergic to honey and maple syrup as well? Is raw sugar okay?