Ugh. Has Anyone Had a Severe Skin Reaction/allergy?

You do need a break.
Hopefully the doctor can get you into the allergist very soon. Untit then give oral Benadryl, the topical stuff does not work as fast on allergic reactions.
Write down what he was doing, what he ate, when he broke out, how severe, document as much as you can.
I have an allergy ridden child, we keep Benadryl and I read everything on the packages, in case I miss something or we find something new.

((((BIG HUGS)))) Today is a new day. Take it one step at a time.

I would wait until tomorrow and give him doses of Benadryl as often as you are allowed until then. Rashes take a long time to go away because your body needs time to purge whatever is causing them. It is not uncommon for a rash to come and go over the course of a few days.

K.
http://www.discoverytoyslink.com/karenchao - got kids? I've got toys! Make your holiday shopping easy by choosing Discovery Toys.

It almost sounds like poison ivy/oak/sumac. I had a friend whose son got a bad case of it in the middle of winter because he carried in firewood that had a piece of a dead poison ivy vine on it. Urushiol can stay active for 1 to 5 years on surfaces. When I get it, the itch will not stop on it's own - I have to go on a steroid series to shut it down. My record was going for 2 months before seeing a doctor about it. I was scratching in my sleep till my legs were bloody and that was the last straw.
http://poisonivy.aesir.com/view/fastfacts.html

I read the other answers, and the other recommendation I have (b/c his rashes have been off and on) is to try using "Free" detergent (free of dyes, perfumes, etc - Tide makes one) and washing all his clothes, towels, sheets, etc in it. I had a horrible rash a while back and making this switch really helped. Also, try the Aveeno soothing itch relief body wash. Lastly, keep in mind that whenever you buy new clothes for him, you need to wash them before he wears them. My derm suspected that my rash started when I wore a new pair of pants before washing them - apparently manufacturers often wash new clothing in fermeldahyde to help them keep thier shape (yuck!)

I have had issues with rashes my whole life. If I sit on plastic I break out, when I go swimming the water irritates my skin and just about everything irritates my skin. The only thing that keeps from breaking out in a rash everyday for just any reason is zrytec. I have been taking it for years and it is the only thing that works for me. Yet it doesn't really help with my allergies... anyways I know how frustrating this is first hand so I hope he finds out what the problem is.

Both of my kiddos have suffered with rashes and skin issues. Back in May, we started them on a supplement which has helped them dramatically with the skin trouble as well as their allergies. Let me know if you'd like more info. Hope the appointment goes well and that your son is feeling better soon!!!

I have this issue with detergents. Honestly just one day my skin said no more and I had to switch to free and clear. It started as a small rash mostly on my stomach. Now I will have a rash and welts from head to toe. Any part of me that touched my clothes is itchy. Even the bottom of my feet!!
It could also be from something he is eating. Allergies come out of no where and gradually get much worse. Benedryl should take care of it. But that will only last 4 hours.Try washing his clothes & sheets and see what happens. Esp if you changed soap, softner or dryer sheets recently.

It sounds like eczema. My son has had it all his life. The typical place that turns up is the back of the legs and other joints. After a bath have him cover himself in Aquaphor. You may have to get steroids from the Dr. to get the big flare up under control, and then maintenance with Aquaphor (nothing w/ dyes or fragrance). It gets worse as we go into winter and the air dries out.

Did he eat something new, a food he has never eaten before?
Did he play in an area that was new to him (re growing plants)?

Have you had him tested for allergies?
If so, have you adjusted his diet and surroundings
according to the results of the tests?
If not, are you willing to have him tested?

I have had allergies for all of my life.
Itchy skin can be torture.

If/when you find his particular trigger(s),
do your best to protect him from exposure.
You did the right things . . . . itch cream and Benedryl.
See if the doctor will prescribe or recommend something
topical for itches stronger than the cream, if necessary.

Please give him a hug for me, if that's OK.
I remember how miserable I've been when I've had itchy skin,
especially over a large area.

Oh . . . ALSO . . . .

Sometimes it's not just one thing that triggers a reaction.
It's a combination of 2-3-4 things happening at the same time.

For example . . . .
I might be fine with just one little cat nearby.
I might be fine if I eat something that I react to.
I might be fine if there's a tree releasing its pollen in my neighbor's yard.
But if all three of them occur at the same time . . . .
DOOM!

===============================
I read some of your previous questions.
It occurs to me that the melt-downs could be another symptom
of the same allergies/sensitivities. Not directly but, for example,
if his body is trying really hard to maintain its comfort level,
if his brain is distracted by physical discomfort,
he's going to be significantly less able to be calm, focused, etc.
There's a better way to express this but it's late and I'm sleepy.