What do I do?

My 9 year old keeps getting hit by this 6 year old. And my son crys. The 6 year old don't seem to care what I say and runs everytime my husband is home. The parents dont speak English and from the looks of it, they can't control the boy neither. Its horrible but I told my son to hit him back to protect himself. But he refuses to do so and just goes in a corner and cries. I can't lock my child up in the house just because someone can't control their child. Please suggestions.