Not meaning to sound crude here at all but BP's defecate more than 8g sometimes so it is really nothing to worry about! My yearling boy can drop 40-50g after doing his business and he is smaller than your BP.
3 weeks is not a long time and your BP could be fasting for the breeding period (he is around the weight when it is possible) or he could just not want it. I would keep offering on your normal feeding schedule and if he starts losing substantial weight then perhaps thinking about other methods would be helpful but sounds like your BP is healthy
Nothing to worry about.
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