I Think my Cat is Dying

He's all skin and bones.  He's like a skeleton.  ㅠ.ㅠ  For the past few days, he's been having these episodes (seizures I think) when his legs give out and he falls out in the floor mewling all pitiful. He won't drink, but he eats like he's starving. Today his poop  was black (okay yeah you didn't need to know that but god he's freaking dying ㅜ.ㅜ) Idk what to do. He's pretty old (11 yrs, but we've only had him about a year and a half) and my mom's been wanting to get rid of him for a while now. My poor kitty  D': Idk what I'll do if he dies, but I'm positive he's going to. :'( Fml