If a 1500 gram snake is going off feed, and losing drastic body weight or tone then you need to take it to a vet, it likely has other issues than just not eating. BPs will go off feed occasionally...perfectly normal. However, they will not lose drastic amounts of weight unless there is another issue. Perhaps they're off feed because of a husbandry issue.
I agree with Deborah. No reason an animal that size has to be force fed if it's healthy otherwise.