16yo old son may have HSP

It is best taking your son back to have this checked out I took ill on the 2nd of December 2015 thought it was just a virus and ended up in on New year's eve kept in for a week was been given pain killers but my joints were killing me had a biopiosy done and it was confirmed the following week that I had HSP. Thankfully all tests on my urine have been clear barr 2 infecions but no blood. The only problem I am finding at the minute is my joints are so sore in my feet and arms and some mornings I wake up with bruises. Just seem to be living of tablets

If your not happy take him to get rechecked.

My second flare up of HSP coincided perfectly with a fractured foot but I had no injury around the time of my first flare up. I didn't take any drugs for the fracture though so I think it was more to do with my immune system overacting to the injury.

There is also hypersensitivity vasculitis which can be set off after a reaction to drugs. Perhaps it could be that? The symptoms are pretty much identical to HSP.

First off, the doctors just don't know what the cause is, period !  There are NO known ways to determine what caused it at the moment. Therefore it is left to us in the forums to speculate until more science is applied to this issue.

In my research it does break out into two main types of issues with each being triggered in different ways.

They type you are talking about sure seems to me to have a high corrolation to allergic response.  The real difficult part is determining the trigger and/or continued contributor to an allergy.   There were so many factors as part of the incident and even possibly related but not considered as part of it.

For example it could be any of the items you mentioned and you can eliminate then and with prednisone, if symptoms begin to clear up and don't immediately come back, then likely it could be due to them.  I really hope they have him on a taper and start it high enough and come down slow enough.   Maybe something like 6 tablets, 6, 6, 5, 5, 4, 4, 3, 3, 2, 2, 1, 1  or something like that.  NEVER just stop taking Prednisone, don't miss a day and please taper it down.  The shock to the system is not good.  

Please also consider everything wholistically.  By that I mean if also due to this instance he ate or was exposed to anything else like a rare type of food or drink that was started around or just before it could also be the cause and a traumatic shock that kicked off the immune system was when the symptoms would first show up.

It took me almost 2 years to finally research enough and try to eliminate enough things such that I found what I was allergic to (chocolate) and Prednisone seemed to work pretty well which also tells me it was the allergy type.

However, this is just one type.

Try to clean and eliminate as many things and see how the Pred goes.  If it seems to work, then I'm guessing allergy to something and hopefully not the metal but any foreign substance in the body is likely to trigger immune response and even if it's not the metal, the slight allergy to whatever (if that is what it is) is going to push him over the edge to have the HSP symptoms.   

Well good luck and let us know how things progress.

Also the dermatologist is the last doctor you should see about this.  It should be an immunologist.