What all goes into these premade solutions?

Obviously each one will have some differences, not asking for anyone's secret recipe.

My son wants to make some of his own bait and trying to decide if it's mostly just glycerin and preservative or what?

If it's a good one it better be secondary odors that increases that attractiveness of the bait.

It's not mostly glycerin, rest assured. There are some darn good ones on the market today- Check out Mr. Jameson's, Mr. Dobbin's, or Dunlaps- or heck, get all 3 and experiment, see what you like and what works best