My West Highland Terrier is being really fussy with his food.He enjoys breakfast cereals, Shreddies.Is it ok?

A few Shreddies as a treat is ok but it won't keep him healthy if this is all he eats. Also dogs are like kids, if you give them treats when they don't eat their main meal then they'll keep refusing proper food in the hope of treats. Try just giving him dog food (try a few different varieties) for a week or so and he'll probably eat sensibly again.