Does my dog have a reaction to his food?

my 8 yr old gsd has had a recurring itch when he goes to his vets they treat the soar skin but I'm sure they have not found the source of his problem. he is a fussy eater but have found he will eat iams dry food and has been on that for about three years (plus a cod liver oil tab a day). his skin itch is mainly on the top of his back and he has no flea dirt or fleas that i can see. it start as a small lump under the skin and then he scratches it and breaks the skin I'm sure this is when it has got worse as the scratching and biting results in an infection.i have an appointment today at my vets so I'm sure they will be able to help him. i wouldn't like to change his food as he eats little anyway and is difficult to keep Waite on