Your dog's leg hair will probably need to be cut each time you groom the rest of him. Every dog is different. Some have short hair on their legs and some have long hair. Some dogs even have longer feathered fur on the back of their legs. You can scissor cut or even trim leg hair with clippers. Dogs with longer hair may need to be cut in a cone shape so their hair is shorter on the top and feathering out towards their paw. Dogs with shorter hair on their legs still need to be groomed for a neat and tidy look.
Your dog may or may not be excited about grooming his legs. Some dogs don't mind if you are touching their front paws because they can see what you're doing, but when it comes to their rear legs or the backs of their front legs, they may be a little nervous if they can't see the tools you're using and exactly what you're doing. Take your time to get your dog comfortable with both the grooming process and the tools you'll be using.