I think waxworms would be fine in my opinion. Oh, and btw, buying crickets and other feeders is a lot cheaper online. I used to buy feeders from petco and someone suggested I buy online. It's a lot cheaper online and you get more. Goodluck with your little guy! :D
Just to let you know, lettuce isn't good for them. It has poor nutrition in it. And, it shouldn't be used as a staple. Good veggies to feed him are collard greens like mustard greens or dandelion greens. Kale is good too. :D
So, you feed him in a 10 Gal? Why not in the 20L? Maybe it's stressful for him in the 10 Gal? I have no clue why he wouldn't want to eat more than 8 crickets. Oh, and putting a hide in his tank isn't a good idea for little guys like him. They often will hide and sleep and not want to come out to...