If you want to adopt this dog, why not approach the owner directly? When you do that understand that another dog will probably be put in the same situation.
If you want to continue walking his dog, you may want to look into "reactive dog" training so that he will not react to people and dogs on the street unless they are a true threat. This training, as I understand it, starts under threshold and works to gradually increase that threshold. Pat Miller is one person with a training program who has written several articles on this topic in Whole Dog Journal.