Those bumps may be hives due to allergies. Stig used to have them. If it's bleeding, you may want to go back to the vet to get some antibiotics to take care of any infection. Is this the first time you've noticed them since the food change? Approx 80% of allergies are due to food and the rest are environmental. I think it's ok for him to change food again as long as the transition is at least a week.
Did the vet mention anything about taking Benadryl? When Stig had hives, he recommended 1 pill of benadryl in the morning and evening meals along with a pill of salmon oil. Salmon oil is great for a dog's skin. He also gave us an aloe and oatmeal shampoo and advised us to rinse him with 1/3 white vinegar and 2/3 water after rinsing off all the soap with water. Don't rinse his face with the vinegar solution though because it'll sting his eyes.
Hope that helps. A lot of people here will also recommend from experience. G'luck in finding the right regimen for him. Don't worry, you will eventually.