so skinny

My 13/14 year old cat is really really skinny but he eats ALL the time and when he's not eating he's sleeping so he's not doing anything to work it off he also goes to the toilet regulaly and drinks regularly can anybody tell me whats wrong my aunty saiid it could be thyroids and he's been to the vet before and they never said anythingso we are going to a different 1 and were getting an appointment today but if any1 knows whats wrong maybe i can change something at home that will make him fatter

if he had a thyroid problems he would look fat mayb hes got worms and you have just not noticed it? (easy thing to do) or you mgiht need to change the cat feed. good luck