try google searching cyclic vomiting syndrome
Pay close attention to what he is doing just before he goes to bed...Is he eating his tooth paste? Is he drinking something frequently that he may be mildly allergic to? Is he getting an over the counter medicine, like a nasal spray, a decongestant, a cough medicine, or baby aspirin? Is he taking a vitamin? This does sound like a reaction /inflammation to something...Does he by chance rise in the night and get something to eat or drink? Get into pet food? I know some of these things sound a bit funny, but I have seen it happen...
Another consideration, he may be swallowing a lot of phlegm at night...and the rash was from being too warm and sweating...If he has a high amount of phlegm, the lump may be a lymph node that is enlarged....If he is fighting a chronic inflammation of some type (food allergy, soap sensitivity, ear or throat infection) this could be it too...Sorry, I think I may have given more questions than answers...Hope it helps..