your dog is probably being aggressive because you have not yet established your dominance. You feel sorry for the dog because it is old and its history etc but what you are doing is not kind to the dog at all.
Once your dog understands clearly who is the pack leader out of you two he will be free to leave you to deal with the other dogs. Try to think of your dog not in human terms but as an animal. His needs are not the same as ours ,he needs to be part of a pack to be balanced and happy .
Watch some Dog Whisperer it will show you how to give your dog some balance and to establish yourself as the pack leader. Then your dog will be a lot happier knowing where he is in your 'pack'
Once your dog understands clearly who is the pack leader out of you two he will be free to leave you to deal with the other dogs. Try to think of your dog not in human terms but as an animal. His needs are not the same as ours ,he needs to be part of a pack to be balanced and happy .
Watch some Dog Whisperer it will show you how to give your dog some balance and to establish yourself as the pack leader. Then your dog will be a lot happier knowing where he is in your 'pack'