Vitiligo or Hypopigmentation?

My son started losing pigmentation on the back of his thighs, under his arms & 1 toe  5 yrs ago...he is now 10.  At the time, ran to numerous dermatologist and 1 seemed to know less than the other. His Pediatrician at the time told me not to worry, that it didn't seem like Vitiligo just yet, (during the winter it's hard to notice, but he gets a nice tan in the summer, and then it is really noticeable) The dermatologist took a skin biopsy on a small spot on his chest, and it came back negative for Vitiligo. Also had blood tests for auto-immune issues which also were negative. They diagnosed it as "Hypopigmentation",  & they threw a script for steroid cream at me & left it at that. Problem is, the areas on back of this thighs got worse over the years...area is lots of large irregular shaped patches all way down to knees now (I stopped the stupid cream thing, because all it does is make the skin slightly pink...not worth the steroid factor or the "everyday application focus on the problem" which upset my son).
Now, the Pediatrician, after looking at it said: "yeah, I guess it is Vitiligo" (great...thanks Doc!) Still have NO answers for this, and don't know which "diagnosis" to believe.   PS: He has allergy-induced Asthma..don't know if that has effect? Grasping for ideas here, obviously...
ANYONE??? Any Dr.'s out there have a knowledgeable/refreshing opinion?  Would really appreciate it. Thanks!