Is it really the responsibility of an HIV-positive person to tell a platonic roommate his status?

No way, it's his right and his privacy. I would HOPE that my roommate would tell me, but I have no right to expect him to. It seems like your roommate only found out recently, so give him some time, he is probably still struggling with the realization and doesn't need any more grief right now. Give him some time and he will tell you. Listen to your mother, she is way more qualfied than the health tech you talked to.