Help! Teenage boy with rash and no one can diagnose!

It seems as though the rash is confined to his lower back/upper buttock, and his thighs?  If this is the case, perhaps it could be irritant contact dermatitis...or allergic contact dermatitis.   I'm thinking that he may be allergic to one of the fabrics in his underwear/shorts that he wears or possibly to an ingredient in laundry detergent..  Try changing to a 100% cotton or organic cotton brief and start washing his clothing in a laundry detergent that has the words "free" or "clear" on the it, as this will mean the detergent does not contain all of those irritant ingredients.. 

I agree with Melissa, have you asked about having patch testing?  Were you aware of a trigger?  I had something similar almost 20 years ago & it turned out to be chemicals in the washing liquid I used for my clothes.  The manufacturer had changed the ingredients, which they tend to do fairly frequently.

good luck.

They found that I was allergic to formaldehyde & formaldehyde releasers, amongst other things.  I started to investigate why I was itching so much &, although these chemicals were in my washing liquid, it was triggered when I changed my car.  I had always driven Volvos & had gone for a Skoda because it was smaller & more economical.

I learned that formaldehyde is banned in Sweden - the Volvos I drove were all made in Sweden.  It is also banned in Japan.  It was not banned in the manufacture of Skodas at that time.  It may have changed now, I don't know.  I now drive Toyotas. 

My rashes were on parts of my body that touched the car's seat, including under my arm, from when I put it over the top of the seat for reversing etc..

This is just to give you an idea of where I found one of the chemicals that I am allergic to.

I have just had a quick look at propylene glycol allergy.  There is a tremendous amount to go through, how did you get on?

The Dermatologist I eventually saw, told me that I wasn't allergic to latex but, formaldehyde was using in the manufacturing process, so I should avoid it.  It is easier to say I am allergic to latex, than to explain everything!  I looked at where I was itching & tried to think of what had been touching those areas.  It was only when I went to reverse my car, that I realised why the back of my left arm was blistering & itching.

The Dermatologist had also told me that formaldehyde was used as a 'dressing' in clothes & I have, in the past, tried on trousers in a shop & then started to itch immediately, another place that they must use formaldehyde, is in air con because I have had the same thing there & so it goes on!

Your son appears to have his rash around the area where he might wear cycling shorts, for example, is that right?  You & he, need to look at the most obscure places & not to think it is just the propylene glycol.  That appears to be part of it.  If you think latex, then remove latex & see how you progress from there.

My Dermatologist gave me a 'very potent' steroid cream.  This has to be used sparingly but it does stop the itching.

Are you in the UK?