Bulldogs are not designed or bred to defend people.
His gonads will not help him defend you either. Only allow him to keep the testosterone that tends to make males more argumentative with others.
The first step with attention is to teach the dog to look at your face consistently when you say his name.
Teach him this with food.
All you need to do is stand in front of him, and show him a treat. Wait until he looks at your face, and he will eventually, and at that time say his name and YESS! and give the treat.
Soon he will be offering you this behavior in expectation of a treat. This means you are making progress.
So now you demand a longer gaze before you treat.
Once the dog learns to look when you ask, then introduce light distractions. the only correction I might use is a little tug reminder using a buckle collar.
When you are OUT and if he refuses to attend to you, I would put a steady gentle but firm straight up pressure on his collar. He will quickly realize that his breathing is more difficult.
As soon as you notice he is focusing on his discomfort instead of the other dog, you become the good guy with the COOKIES. THIS is when you ask for his attention and turn him away from the other dog.
I would also carry something with me to keep other dogs away. If you can, I would warn other dog owners in your area that you will no longer tolerate loose dogs running up to yours, and that you are prepared to defend yourself and your dog from those dogs that are running loose.
I would then walk with a keep riding crop (my favorite) or a bottle of vinegar water with 2 drops of tabasco in it. This one works well, and they generally don't come back for a second dose.
I would strongly urge you to reconsider you decision not to neuter your dog.
Dogs who are not going to be used for breeding are much better off altered. It will completely remove the sex drive from his life, and he will have that much more time to focus on you, being your companion, and loving you.
And PS. One of the toughest dogs I EVER had was a neutered male. If your dog is past a year of age, neutering will not affect his temperament very much except to decrease marking behavior and aggressive posturing towards other males. These behavior changes happen gradually over about a years time following the neuter of adult male dogs.
Good luck, and if you have other questions, let me know.