I'm no expert on Jackson's but as far as I know, this is a sign of overheating. Chams open their mouths to release heat sort of like a dog pants. How are your temps? Basking? Does he do it under the basking light?
Montanes, such as Jackson's, are known to eat very little. He is still growing, so I'd try to make sure he eats everyday. But as he gets older, he will only need to eat every other day; or even every two days. Are crickets the only thing you're feeding your Jackson's? If it is, try offering some other pray items (ie. Silkworms, Butterworms, Hornworms, Flies, Moths, and WC insects [from pesticide-free areas, of course!])
As people have said, try lowering your basking temp to stop him from 'gaping'.