You need to understand the difference between urinating just to empty the bladder and urinating as a form of communication for dogs.
What your dog does is communication. He takes a nice walk, smells the urine or feces of other dogs and puts his own smell out there for other dogs to smell him. For a dog these smells contain information about the individual dog that left it there, its health condition and sex. Covering another dogs urine with their own is also a sign of dominance.
This urinating in small amounts and smelling the urine of other dogs is like reading the newspaper in dog-style. Who was here with whom and was there a nice girl around?
Lifting a leg even if there's no urine left is a very common behavior as well, especially in males.
This is the natural behavior of dogs and you should not reprimand your dog for it.
Do his buisness
If you want your dog to do his buisness quickly in one go, walk to the nearest tree or patch of grass where he is allowed to pee or poo. Wait there until he finished his buisness and return home afterwards (or after 10 minutes if he doesn't). Always go to the same spot and don't walk any further.
After a few times your dog will realize that this is not a walk to let him "read the newaspaper", but to let him relieve himself. He will learn that this spot is his toilet.
It's also a good ides to do this "toilet walk" always at the same time (e.g. first walk every morning or last walk every evening) so your dog doesn't get all excited over taking a walk only to be dissappointed.
Please don't cut his walk time short just because he learned to do his buisness quickly. Taking a walk is a very important and happy part of your dogs every-day life.