If it's chickenpox take vitamin D at the rate of 8,000 units a day! a cool or lukewarm bath with baking soda added may help to relieve symptoms, such as itching.
If it's german measles or the pox make sure he stays away from pregnant woman!!
Did his doctor do a blood test for Rubella?
Hi if it is german measles they cover your hole body and tell your son not to rub his eyes they can make you blind. My daughter had them when she was 6 month old, and she just screamed for 2 weeks then lots of spots came out. they covered her hole body it looked horrid. The doctor said to stay away from anyone who was pregnant. If it is chickenpox they blister and then scab over and they itch like mad. What i can remember of my daughter her spots did not scab they just went. Hope your son gets better soon.
Sorry i forgot to add if you get chickenpox when you are older they can turn into Shingles it is painful red spots that spread in bands over the body especially around the waist. If you think this is happening i would see the doctor again.