Moto: how long has he had the runs now?
Food allergies CAN pop up suddenly as the dog matures. I had the Doctor that I work for caution me about switching foods all the time because not only is hard on thier system, if a food allergy DOES crop up later it is that much harder to diagnose.
Have you tried a bland diet on him yet? This is what we reccommend for doggie diarrhea. the formula is feed 1 part cottage cheese or other mild protien source like tofu (not hamburger!!) to 3 parts white rice (not the minute kind, slow cooked) for a few days until you see improvement. You feed 1/2 the proportions he would normally have, only twice the amount of feedings. So, normally he gets 2 cups of food a day, 1 in am, 1 in pm. you should feed 1/2 a cup of the bland, 4 times a day.
If you arent seeing improvement, something else is going on. you should run a fecal next, to check for not just worms, but parasites like giardia and coccidia. they only show up under the microscope and cause diarrhea and digestive upset.
If the fecal comes up negative, then it is probable to say that we are dealing with a food allergy.
Sorry about the lecture, i hope that it helps!