My Collie lets the neighbor's dogs come and eat his food

[Wolfram Richards]

My Border Collie is now 9 years old. He is kept as the family dog and he is the only dog we have.

When he was about 7 years he started this thing where he would not eat the food given to him. The food we give him isn't in anyway foreign to him, he just wouldn't eat it. 

Because of that the neighbour's hungry dogs come over to eat his food and he sits while they help themselves to his food. When one of us comes outside he runs and chases the dog away, but otherwise he watches the dog eat. 

Is there a reason for this behavior?