IMO - it is worth the money to do the allergy testing. It could save the dog's life at the most; at the least, save him alot of discomfort. We fought a two year battle with Nina to try and figure out what she was allergic to and then to control them so she could live comfortably. What put me over the edge with finally caving for the allergy test was when she went outside to go potty and came in looking like she had gone 5 rounds with Mike Tyson.
She was outside for 2 minutes at the most. Her eye had swollen shut and she was coughing. She had been stung by a bee. The fear of anaphylactic shock motivates you.
Nina always had ear issues - the vets immediately suspected food allergies. If the vet does suspect food allergies, you can ask them to put the dog on an elimination diet to try and find out what specifically he is allergic to. Corn is a big one for rotties; it is present in alot of foods. Grains in general tend not to agree with them.
Good luck with your guy.