I'm assuming he meant 90 HSC mark in which case it would be equivalent to 90 HSC mark this year. If he meant 90 raw, we can't tell the conversion because different tests are easier/harder etc
You don't find out what your mark in the paper is unless you pay for it. Are you sure he meant this? If so, then maybe 75 would be the equivalent raw mark but the 90 that is reported to you is the HSC aligned mark not the raw mark.