Hi K.,
Childhood acne is very rare and is a red flag that something serious is going on in his system. The skin is a true portrait of what is going on in the body.
The acne doesn't need to be tended. You need to find out what is causing it. The dermatologist was a good place to start but he obviously is not looking at your little one holistically. My first step would be to see a Naturopath and see if they can determine what is going on. They read blood work a little bit different than a doctor does. I would also get several opinions.
I would also suggest you start removing toxicities in his life. Fast or processed foods can be an ugly culprit. Synthetic chemicals in the home can cause issues as well. A good absorbable multivitamin/mineral complex will always help. You need to get him to a baseline so the diagnoses is going to be accurate. This is what helped our family the most with our basic health.
I was the sickly kid that the doctors poked and prodded all my life and when they finally figured out what was wrong, they treated it the wrong way too. Be proactive and aggressive. The body is complex but it is also very simple. Make sure you understand everything they are testing for and doing.
Hope this helps! God bless,
M.