ex husband's injury

I have been divorced from my ex for three years. He was and is the most selfish, insensitive person I have ever met. I have not missed him one minute. Recently he was in a serious accident and needs help getting to the doctor and his therapists, getting groceries, doing laundry, and that kind of thing. He has no family around, and has run off friends. Basically I am all he has. We have a teenaged son who is asking me to please help him out. I can't explain to my son how miserable of a man he is (although he knows a lot) because I don't want to belittle his father.
I need to know what is the right way to establish a boundary or if I should just help him out to set a model for our son. I honestly feel my ex will not be appreciative, but that he will feel entitled. I hate being around him.