I would be a bit skeptical of that reduce the cat food diet unless you have a high fat content food? Can you post the first five ingredients and the protein and fat content of his current food? a bit over 300g's isn't that much my Feral is still fairly small and only 30gs under 300gs. It is true though some can be over weight yet you mentioned he doesn't have the signs of a overweight hedgehog.
As far as i know crickets are low in fat and provide good protein and other needs of a hedgehog. Fruits and Veggies can be found Here
I am no expert and would definitely wait for our vets of hedgehog ownership and raising to speak but I would not reduce the cat food diet to greatly that from everywhere I have read is very essential to their diet although I could be wrong :P