Apparently it is an allergic reaction; if your son has taken motrin before with no ill effects it isn't likely he would suddenly develop a reaction now; if you noticed a reddening of the tongue or mouth lining soon after his ingestion you should immediately seek professional assistance according to the Googled sites I've just scanned.
The reason this story caught my attention is because our son has apparently developed an allergy to Motrin. I had read a story about SJS a year or so ago, but I believe the cause in that story was an antibiotic or phenylbarbitol(sp?). Our son was having hives earlier in the year, and we thought it was due to something he ate. Then we thought it was due to a tooth infection. (he chipped a tooth. initially, there was no detectable problem with the roots.) The infection was treated, and the hives/edema stopped. Until we gave him Motrin several months later. Soooo, we do not know for sure that Motrin was causing his hives/edema earlier in the year, but we do know that his eyes swelled shut the last time he had it. It was very scary every time he swelled up. I took him to the doctor one of the times, and she thought it was an allergy to something in the environment and that it would go away. She told us to give him Benadryl. Anywho, our son has taken ibuprofin with no problem until the last year.