check the width of your cham's forehead.
feed only crix that is smaller or equal to the size.
as far as silkworm, small to medium size will be good.
Phoenix worm is small already, so get the med size.
I say hold off on hornworm until he's a bit older.
Hornworm can put quite a fight (they spit). so, it might freak young chameleon. Plus, they attach to vine real strong.
but, just buy a small one first if you insist.