Sea Change by Iain Rowan
A teenage boy, John, who is always picked on in school goes to visit his sister for a brief visit to help him get over a tragedy at school.
He meets two friends, a brother and sister, who want to spend time with him and enjoy his stay. In addition, he also meets a strange old man who seems to think he has a special power and needs it for him to live. In a confusing mix of streets, John also sees a mysterious black dog. Afraid everyone will think him nuts, he is able to find a book about local folklore and legend, only to find out the man and the dog are from days gone by.
FInally finding the courage to stand up for himself, he finds his true friends will also help him try to defeat the old man and end his cycle.
You’ll need to read it for yourself to see if John wins or the old man continues to find another person to help him live.