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

I can only speak for my Bait Solution. If you use a pint of my bait solution, you will have 2 ounces of glycerine in your gallon of bait. That's why you'll need to add more glycerine in colder climates to keep it from freezing.
When I make my Beaver Medley using my bait solution, I have to be careful that my couch hound isn't around when I'm cleaning out the bucket. He'll dive in and start gobbling up the leftover morsels. If he eats enough of it, he gets the runs, then I'm in trouble with the missus.

Thanks for all the replies.