I would definitely get into a Naturopath and Dermatologist as soon as you can. Poor little guy!
Is he in good spirits, normal behaviour etc otherwise? Does he have any other symptoms? I would be more concerned if he also had a fever or puffy eyes for example, or if the rash was turning to blisters.
Tell your Dr. he is scratching at home, don't down play it. I would like to hear what he would diagnosis it as! Maybe even go to a walk-in clinic if you can and see what another Dr. would say.
Also, again, I'm no doc, but I believe petechiae are small pin-head like tiny broken blood vessels, (sometimes from straining, severe coughing, vomiting - I have done much research on this b/c I have had them all over my torso since my first delivery). When you said he woke up with a small petechiae "the size of a quarter", did you mean one big one, or a cluster of small ones? If it's one big one the size of a quarter, that sounds more like Purpura, which can be a symptom of something much more serious. Purpura are larger areas of bleeding under the skin, (i.e. bigger than 5mm). If he has Purpura and a fever, I pesonally would take him to Emerg. Not to alarm you, but Petechiae that appear during illness, particularly with a fever can be a sign of infection in the blood, which can escalate quickly and could mean Meningococcus. Sudden onset of Purpura are not normal.
Note: if it's petechiae, (the small ones - 1-5mm) it's less serious, and this apparently can also be caused by an allergic reaction.
Interesting, taken from Motrin's website on allergic reactions to Children's Motrin.
Allergy alert: Ibuprofen may cause a severe allergic reaction, especially in people allergic to aspirin. Symptoms may include:
From Healthsquare.com, on Motrin (I added bolding):
What side effects may occur?
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Side effects cannot be anticipated. If any develop or change in intensity, inform your doctor as soon as possible. Only your doctor can determine if it is safe for you to continue taking Motrin.
More common side effects may include:
Abdominal cramps or pain, abdominal discomfort, bloating and gas, constipation, diarrhea, dizziness, fluid retention and swelling, headache, heartburn, indigestion, itching, loss of appetite, nausea, nervousness, rash, ringing in ears, stomach pain, vomiting
Less common or rare side effects may include:
Abdominal bleeding, anemia, black stool, blood in urine, blurred vision, changes in heartbeat, chills, confusion, congestive heart failure, depression, dry eyes and mouth, emotional volatility, fever, hair loss, hearing loss, hepatitis, high or low blood pressure, hives, inability to sleep, inflammation of nose, inflammation of the pancreas or stomach, kidney or liver failure, severe allergic reactions, shortness of breath, skin eruptions or peeling, sleepiness, stomach or upper intestinal ulcer, ulcer of gums, vision loss, vomiting blood, wheezing, yellow eyes and skin