I`d say at his age crickets are probably still your best bet. wax worms and butter worms are always reserved as treat worms. Mine likes the butter worms slightly more but the wax worms are typically half the cost.
Horns and silks are much bigger, cost more, and cost to feed as they eat a specific pre-mixed chow, as opposed to other feeders. I like to give him maybe one of these a week, as I don`t keep their food and can pick up a horn for a dollar or a silk for 65 cents. They`re cheaper in bulks but for my treat purposes I don`t mind picking them up in singles. He nearly craps his pants when he sees these, it is like oh.. my.. god.. you are.. amazing.. chomp chomp chomp.
I like to feed supers to my 7 year adult, I find they digest easier than mealworms regardless of the size. Mealworms have a harder shell, but the shell issue aside I found its more the meat-to-shell ratio. Being that for them to get a decent portion of `meat` he only needs to eat so many, resulting in only so much shell consumed, as opposed to if I tried to attempt just mealworms he would eat so many more just to get a small amount of meat, and a high amount of shell consumed.
The mealies are cheap though so it is fun to mix in especially when they are bigger and can handle.
I`m at the point now though for bulk pricing I might just buy the supers, he seems to prefer them more regardless.
So sorry, I think I got off on a tangent. Depending on your region I hear roaches are a great alternative to crickets. Breeding would be your best bet at that point, dubias seem to be the popular choice, although discoids are up there(not sure of discoid size)
So to conclude I would probably continue with the crickets, just buy them bulk online. When he gets past 16 inches you could test out supers, only introduce them slowly into the diet and keep an eye on their stool for excessive amounts of shell. If you see large pieces of shell that means they had to pass it without actually digesting it.
Dont worry, with his size it sounds like you`re almost out of the `must be` crickets zone. Youre right that at this point it doesnt seem like the mealies make any sense, Im probably going to cut them out altogether myself.
Theres a local store by me that sells in bulk, its great on cost, but you might not be so lucky.. I would try searching for a local reptile exclusive shop. Pet store chains like petco and petsmart even my nearest big als aquarium chain likes to rip people off for feeders. Seeing this kid at the register once talking about how he didnt know how he could keep buying cups of these things like daily(per the feeding requirements of a baby) and just thinking to myself there is a much better way.