My 3 year old is always sick and tired until he eats?

For about 6 months my son has had stomach pain and is always tired and grumpy with bags under his eyes and even swollen eyelids that make his eyes look sunken, fevers. He has had blood and urine tests and besides being borderline anemic all tests are good. We thought he was diabetic cause we noticed he is more happy and active if he eats alot compared to when he doesnt want to eat at all, Diabetics runs in the family to but the drs say the blood is fine. Any direction you can send me would be great Thanks so much!