I wouldn't worry about cutting them up, he's big enough to take whole worms no problem.. Plus i'd just run to walmart if there's one by you, that's where i bought all my worms. Right out of the bait cooler..lol
The reason for chopping them up is to get him interested and alert his taste buds into action seen as he is not eating as he should be. Once the ray is on them, then feed them whole. Also try putting a few prawns in the juice of the worms to get him interested in them to.