Why is my rabbit going bald?

I have left my rabbit in the care of my sister for a few months and i#m not entirely sure weather she has been caring for him right. it's a rex rabbit and when i went 2 check on him 2day he had bald patches on both his front legs. it's strange coz there's small tufts of hair on the bald spots but i can't understand why he's losing hair can anyone help me??