Why isn t my cat eating? Should I worry?

I have a 9 year old indoor only cat. He is on the heavy side so he s been put on a diet where he is fed in the morning and evening and given a set amount of food. Normally he chows right down! But today he s nibbled a little and not really eaten. Should I be worried by this? He s eaten all his wet food and drinking normally. He has eaten a very small amount of dry food.