4 year d boy with pimples?
Have you tried eliminating dairy? Citrus? I know there are a couple more major allergens that you might want to test. If you post in Health & Healing, they've got an allergy section, with some really experienced parents who might be able to point you in the right direction.
i would still try eliminating dairy.
i would increase his beans or lentils intake - quinoa or nuts too - all high in protein.
milk is really big on low impact allergies.
taking out gluten and dairy from diet (actually just reducing them) has had a huge impact on dd's issues. we are mostly vegans and eat meat maybe once a month or less.
Since they can't actually test for intolerance (my dd has some food intolerances), could you try an elimination diet and see how that works? It may or may not be dairy related, but it does sound like it's diet. Keeping a food diary and doing the elimination process can be long and seem like a waste of time, but in the long run, it will not only help his skin, it could help with his immune system and a host of other issues that may crop up over the years. Good luck!