Do you cup feed them? If he absolutely refuses to eat them, just wait for them to pupate and turn into flies. He won't be able to resist the flies!
Biggest avail? could be a little too big, a bit intimidating?
Try "sticking" one to the side of your enclosure and waiting quietly to see what happens.
A lot of times my chams are super excited for feeders when offered that way. Once he eats a couple like that and knows what they are, you can return to bowl feeding.