Eating only one kind of food makes a cat fussy. Sure he'll have a favourite. If I had the choice of smoked salmon or a steak I'd personally opt for the smoke salmon but eating that every day isn't very practical!
Cats are clever and some times a cat won't eat knowing their human will give in and provide them with a more delicious favourite dish.
If your willing to buy him the same flavour all the time i wouldn't see it as a problem but a fussy cat whos always had his own way isn't going to be aggreeable to eat anything else (not good especially if he ends up needing a specialist diet when he's older)
My advice be firm and give him a variety, you never know he may even enjoy it more than the same thing!