Well we are almost in March and it could very well be a virus.
But with my experience (3 children total ranges in ages 4, 2 and 6 months), it could be Roseola. Normally with that though (it is a virus) you don't see the rash appear until after the fever has broke. Now every kid is different and we aren't doctor's here. When my youngest had it, he developed the rash when he still had the fever, but his fever broke within 5 hours after the rash appeared. How old is he? If he is very young, I would go to Urgent Care tomorrow morning or the ER tonight because you cannot have him get dehydrated. If you give him Motrin/Tylenol, does the fever go down at all? Teething does not cause a rash on the face, but the fever can vary. I was always told by my kids dr that 99-101 is normal for teething, but anything above that could be infection or viral. Keep a close eye on your son tonight. It may be a night of no sleep, but better safe than sorry. If he was teething, he would be uncomfortable usually.
Good luck and be sure to call the dr.