If it is gravitational acceleration then it it is positive in downward and negative in upward direction..if it is not gravitational acceleration then it is depending upon the value of acceleration.
Sorry to say,but your bf might found someone new,that's what i have experienced. Talk to him to what you feel about him lately.I know woman has a strong intuition especially a relation with someone dear to her,especially changes he doesn't doing before.i hope you can talk it over with positivw result.