My dogs wont eatone is a new dog i got 3 days agon and the other iv had a month?

recently got a rottweiler frm a pound,,beautiful dog bt very very thin.the first nite i got him he ate all his food but since den (3 days) ago he barely eats 2 mouthfulls,my pup always ate her food but now wnt eat a thing either.i havent changed the food or anything,,please help coz im really fearfull dat the new lil(well big) guy will get sick,because hes already underweigh.hes 2 in october and she(beagle x-we tink)2 1/2 months old