What's wrong with my cat?

My cat is around 16 years old, male. about a month and half ago, my family went on vacation for 2 weeks, and we had someone to take care of pets. when i came back my cat had lost a lot of weight and wasn't as mobile. we took him to the vet, and all the blood work came back fine, however he was constipated a little. recently, he's still been lethargic, though he's been eating and drinking water normally. even though he eats his normal amount of food( maybe more) he is still super skinny. the most serious issue though is that within the last 2 weeks, he looks like he's lost alot of strength in his hind legs, as he struggles to walk on them, and his backside sometimes slides around on the floor while his front legs sort of pull him. is there anyway you could tell me whats wrong with him? also, he's an indoor cat.