I was hoping to get some responses back before giving you advice, but it really sounds like you need help now.
If he isn't fixed, have that done immediately. Have your vet check for a thyroid condition with a thyroid panel. Often an issue with the thyroid will lead to sudden aggression.
If it is not related to a medical issue, then he is going to need a lot of obedience training. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
You need to work with him until he is listening to your commands consistently before taking him to a group class to finish training. Buy a basket style muzzle. They are open and do not constrict the muzzle and thus allow a dog to breath normally and eat and drink pretty normally but he won't be able to bite other dogs.
Often an owner will make the situation worse by worrying about what the dog is going to do. The worry transmits down the leash to the dog and they pick up on that feeling you have and they think it is the other dog you are worried about so they can become aggressive to protect you. So the muzzle will help you feel less anxious and that may help the situation as well.
Another method used is the BAT method. You can read about that here:
Of course, I would not have the dog off leash if he is not in an enclosed area of your own yard and I'd keep a basket muzzle on him if he is around other animals or even strange people to be on the safe side.
If it is a thyroid condition, medication may correct the situation pretty quickly.
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