Yes, I already took him to the vet last Saturday and the vet diagnose him with serious heart and kidney problems which result in his thin frame. His fur coat was in good condition, no ticks were found and his heartworm test was negative. So, I don't think he's neglected but he needs his medication and that's why it's urgent to search for his owner.
Okay, at least I know he is in good hands. I sincerely hope and pray you find the owner quickly - although sometimes there are heartless people who abandon ageing pets instead of taking better care of them. >.>