First, food aggression is normal for dogs, so please don't think there is something wrong with him. Our jobs are to change the behavior. Of all aggressions, I'll take food aggression over anything else.
Fortunately my friend Deb has a great article http://k9deb.com/foodguar.htm
Kathy Diamond Davis is superb with timid dogs, good with aggression. Her article is also helpful on tips::
If you follow Deb's article and you don't see results, I agree a trainer is in order. But most of the time, unless your dog goes Cujo with people over food, early intervention can retrain food guarding.
That said, take a look at the whole picture. You also need to work seriously on obedience, particularly the "LEAVE IT" command. I pair LEAVE IT with "LOOK AT ME". When a dog is taught to STOP and look you in the face, it breaks their fixation on whatever the issue is, gives their brain time to engage, and you time to intervene. Few sites (actually none I have found) are more simple and gentle than Mary Woodward, and no you don't have to use a clicker to do them. : : http://www.clickerlessons.com/