96 grams down to 89 isn't overly alarming, but it is not the way that his weight should be going. When you call the shop you got him from, I'd try and find out if they know where he came from, ie was he wild caught, captive hatched, bought from a wholesaler (which is like flipping houses but with snakes - so no quarantine or close attention to health) etc. Honestly that's really the only useful info you will get from the store. Other than that, he may need a vet. Get a fecal test done first, parasites will sap their nutrients and energy as well as lead to other issues like anemia and GI irritation/ulcers.

Other than that make sure *ALL* his husbandry is on point. A picture of his set up may help us give you some tips as well. Once his environment is settled in and you get him on rats he should steadily improve, if he doesn't there is something more off than just husbandry and new home stress.