My pet dog has been ill for a long time. He has grown old and weak. He has a poor control over his bladder and bowel movements. He suffers from arthritis in both legs which makes it difficult for him to jump, run or walk for good amount of distance. His appetite is good, however the amount of food, regular medical check ups and medicines required for his better recovery is beyond my financial budget. He often develops ulcers in his genital area due to age related soft skin, and random accidents and these wounds require wound cleaning twice daily with expensive ointments and off and on vet visits . overall i know that he is going through senile changes and needs more care and attention from me as an owner which I am not able to give due to job and family-related responsibilities. Some acquaintances advised me to give euthanasia to him. 

Could you please guide me regarding this? is it permissible in Islam? Is there any ruling on this?