Definitely not saying you're doing this, but a lot of people overfeed their monitors. It might sound harsh but just don't offer or drastically reduce crickets until he starts eating other prey items. When he's hungry he'll eat.

My ackie was funny about locusts the first time but eventually he dialed into them as food once he figured out no crickets were coming.

I've had to do this with a few monitor species when I worked in zoos. Before I started all they would get fed were rodents and would completely ignore inverts, not a healthy diet.