hmm, i just remembered, he could be an N, probably.. he is prone to abstract thinking. but acts simple, friendly (?!?)
ENTP? Hm, but I aren't they like....... hm..... not-friendly?? (i don't know any in person). this one is too SOFT, never angry if we don't know something. don't expect much. and yes, he's always late on classes for about 20-25 minutes.
All the ENTP's I know are friendly. And as was mentioned before, they do have tertiary Fe. I still say ENTP.
He sounds either INTP or ENTP to me. Math prof just doesn't sound like a typically ISTP thing, and the ISTPs I've know have tended to be far less nice (or 'soft,' as you put it) than what you described.
really? that's so cool if he's intp... but he's fairly emotional?? he really made pretty personal relationship to students in only 10 days...
p.s. he's not math professor, he is a doctor, finished medicine, and is scientist and (don't know why) teaches "medical statistics"